Equations in One Variable I An equation in one variable is an equation of the form f (x) = g(x) where f (x) and g(x) are functions. Examples. • x2 − 3x + 2 = loge (x) is an equation in one variable. Both x2 − 3x + 2 and loge (x) are functions. • x − 3 = 2 is an equation in one variable. Both x − 3 and 2 are functions. The latter is a constant function. √ • x = 15x − 4 is an equation in one variable. • x2 +7 x−1 + 27x − 3 = ex 3 −18 is an equation in one variable. All of the examples above are equations, because they are mathematical formulas that use an equal sign. The are in one variable, because there is a single variable used in each equation. In all of the examples above, x is the only variable. We’ll continue to use x as the only variable for a while, but keep in mind that it’s just a variable, and that we could use any variable instead. For example, there really isn’t any difference between the equation x − 3 = 2 and the equation w − 3 = 2. The only difference is the name that we gave the variable, x or w, but otherwise the equations are the same. Domain of an equation If f (x) = g(x) is an equation in one variable, then the implied domain of the equation is the set of all real numbers that are in both the implied domain of f and the implied domain of g. Examples. • The implied domain of the equation x3 − 2x + 1 = 3x − 9 is R. That’s because x3 − 2x + 1 and 3x − 9 are each polynomials, so they each have R as their implied domains. √ •√ The equation x = 4 has an implied domain of [0, ∞). That’s because x has an implied domain of [0, ∞), while the constant √ function 4 has an implied domain of R. Therefore, the implied domain of x = 4 is the 51 set of all real numbers that are√in the set [0, ∞) and in the set R, or in other words, the implied domain of x = 4 is the set [0, ∞). • The implied domain of the equation loge (x) + loge (x − 2) = 1 is (2, ∞). The implied domain of the constant function 1 is R. The implied domain of loge (x) + loge (x − 2) is a little more tricky. We can only take the logarithm of a positive number. That means that we want x > 0 and that we want x − 2 > 0, or equivalently, that x > 2. Of course asking that x > 0 and x > 2 is the same as just asking that x > 2, so the implied domain for the function loge (x) + loge (x − 2) is (2, ∞). √ 3x is [0, ∞) − {5}. We • The implied domain of the equation x = x−5 can only take the square-root of a number if it is greater than or equal to 0, so x ≥ 0. We can never divide by 0, so x − 5 6= 0, or equivalently x 6= 5. The set of all real numbers x with x ≥ 0 and x 6= 5 is the set [0, ∞) − {5}. √ • [3, 5) is the implied domain of the equation loge (5 − x) = x − 3. We can only take the logarithm of a number if it is positive, so 5 − x > 0, or equivalently, 5 > x. We can only take the square-root of a number if it is greater than or equal to 0, so x − 3 ≥ 0, or equivalently, x ≥ 3. The set of all real numbers x with 5 > x and x ≥ 3 is exactly [3, 5). Sometimes, you will be given an explicit domain for an equation. For example, you might be asked to examine the equation x2 − 3x + 1 = ex for x ∈ [0, 10]. This means that the domain of the equation is the restricted domain [0, 10]. The domain is restricted, because the implied domain of the equation x2 −3x+1 = ex would have been R if we hadn’t been told otherwise, but we have been told otherwise. We are told here that the domain of the equation is just the set [0, 10]. Solutions of equations If someone writes an equation such as 3x − 2 = 4, they are not saying that 3x − 2 is the same function as the constant function 4. They are clearly not the same function. One is a constant function and the other one isn’t. Rather, they are asking you to find the set of all numbers that can be substituted in for x to make the equation true. For example, we can substitute 2 in the place of x and we’d be left with the equation 3(2) − 2 = 4, which is true, as you can check. The equation usually won’t be true for every number that you substitute for x as we can see by substituting the number 0 in the place of x. 52 That would leave us with 3(0) 2 = 4, which is clearly false. The numbers Thatcan would leave us with 2 = 4,anwhich is clearly false. Thesolutions numbers that be substituted for3(0) x to−make equation true are called that be substituted for x to make an equation true are called solutions of thecan equation. ofIfthe equation. f(x) = g(x) is an equation in one variable, and if the equation has the If f (x) = domain, g(x) is an equation variable, and = if g(x) the equation set D as its then the set inofone solutions of f(x) is the sethas the set D as its domain, then the set of solutions of f (x) = g(x) is the set — S = { α ∈ D | f (α) = g(α) } Returning to the example above, if S is the set of solutions of the equation to the2 example above,S.if SWe’d is thespeak set ofthis solutions equation 3xReturning 2 = 4, then solution as 2 isofa the of E S, but 0 3x − 2 = 4, then 2 ∈ S, but 0 ∈ / S. We’d speak this as 2 is a solution of 3x 2 = 4, but that 0 is not a solution of 3x 2 = 4. 3x − 2 = 4, but that 0 is not a solution of 3x − 2 = 4. Examples. Examples. Suppose that S is the set of solutions of the equation √ = 4. We • Suppose that S is the solutions thesoequation x = 4. We saw above that this equation hasseta of domain of [0,ofoc), saw above that this equation has a domain of [0, ∞), so √ S={ce[0,oo) I=4} S = { α ∈ [0, ∞) | α = 4 } √ = 4. However, —10 S since We see that 16 E S, since 16 e [0, oc) and /i We see that 16 ∈ S, since 16 ∈ [0, ∞) and 16 = 4. However, −10 ∈ / S since √ 4. —10 0 [0,oo), and 25 5, since /E 4 7 −10 ∈ / [0, ∞), and 25 ∈ / S, since 25 6= 4. — — — 0 . Now suppose that S is the set of the solutions for the equation x 2 2= that S is the the solutions the equation x = x. Both x 2•2Now and xsuppose are polynomials, so set theofdomain of the for equation is R and Both x and x are polynomials, so the domain of the equation is R and S = {c ER a 2 S = { α ∈ R | α2== α } Then 0, 1 E S because 0 and 1 are both real numbers and 02 2= 0 and 12 2= 1. Then 0, 1 3∈ SSbecause and 1 are both real numbers and 0 = 0 and 1 = 1. However, because0 32 0 However, 3 ∈ / S because 32 6= 3. 51 53 x x 1-”-••, 1-”-••, Do Do . Let’s look at • look at . Let’s look equationLet’s 2 —x = +at9 an an equation with a restricted domain. Let’s look at the an equation aWe restricted domain. Let’s at the equation a restricted domain. look at the 0 for are told x E with oo). (0,with here Let’s that the look domain is 2 equation equation −x =for 0that for ∈are (0, We the domain is 2 =9 0 are are x Exwe(0, oo).∞). Weinterested toldtold here thatthat the domain + 9+means (0, oo),—x which only in here finding solutions tois the (0, (0, oo), ∞), which means only interested in finding to which thatthat we we are interested only in finding to the equation if means solutions those areare positive numbers. That is,solutions ifsolutions S is the setthe of equation if equation if those solutions are positive numbers. That is, if S is the set of solutions those are positive numbers. That is, is the if S set of solutions, then solutions, solutions, then then 5= { (0, oc) +9= 0} 2 ∈ positive (0, + 90} =that 0 } are also roots of the 5=Sset (0, oc) ∞) | −α +9= {= of{ αall Notice that S is the numbers Notice that Notice that is the of+ all numbers are also roots of the isS the set set of all positive Spolynomial numbers thatthat also roots of the quadratic 2 —x know 9 =positive 0. We how toare find roots of quadratic 2 quadratic quadratic polynomial −x 0.+9We how toOf find roots ofroots, quadratic polynomial 2 —x know =9—x 0.= We how to 3.find roots of quadratic + 9+of polynomials, and the roots 2 and areknow —3 these two only 2 polynomials, polynomials, and the roots of −x + 9 are −3 and 3. Of these two roots, only and the roots of 2 —x and Of are —3 3. these two roots, +9 only 3 is in the domain of the equation. That is, only 3 is positive. Therefore, in 3 is3S={3}. domain is the in the domain of the equation. That is, only is positive. Therefore, of the is, only equation. That 3 is3positive. Therefore, S={3}. S = {3}. 54 The equation √ ==x x++5 5has has an implieddomain domain of [0, oc),since since we •••The Theequation equation x = x + 5 hasananimplied implied domainofof[0,[0,oc), ∞), sincewewe can’t takethe the square-rootofofa anegative negativenumber. number.We Wecan cansee seefrom fromthe the graphs can’t can’ttake take√thesquare-root square-root of a negative number. We can see below graphs that the below,that thatthe thegraphs graphsofof and andx x++5 5dodonot Thatmeans notintersect. intersect.That meansthere there below, graphs of x and x + 5 do not intersect. That means there isn’t a number α √ isn’ta anumber numbera awith with%/ Thatis istotosay, therearen’t aren’tany solutions say,there +5.That anysolutions %/==a a+5. isn’t α = α + 5. That is to say, there aren’t any solutions of the equation with theequation ofthe equation ==x x++5.5.The Thesolutions solutionsset setis is0.0. of√ x = x + 5. The set of solutions is ∅. // Dorn Dorn An importantgeneral general exampleis isif ifp(x) p(x) is a polynomialand and our • ••An Animportant important generalexample example is if p(x)is isa apolynomial polynomial andour our equationisisp(x) p(x)==0.0. Then Thenthe thesetsetofofsolutions solutionsforforthis thisequation equationis isexactly exactly equation equation is p(x) = 0. Then the set of solutions for this equation is exactly the setofofroots roots of thepolynomial polynomial p(x). We how to findroots roots of any We knowhow the theset set of rootsofofthe the polynomialp(x). p(x). Weknow know howtotofind find rootsofofany any linearand andquadratic quadraticpolynomial, polynomial,but butit itis isextremely extremelydifficult difficulttotofind findroots roots linear linear and quadratic polynomial, but it is extremely difficult to find roots ofmost most polynomials. AApolynomial polynomial equationp(x) p(x) = 0 is aboutasasnice niceanan ofof mostpolynomials. polynomials. A polynomialequation equation p(x)= =0 0is isabout about as nice of equationasaswewecan canhope hopetotohave, have,and andeven evenfinding findingitsitssolutions solutionscan canbebea a equation an equation as we can hope to have, and even finding its solutions can be a nearly impossibletask. task. nearly nearlyimpossible impossible task. QQ 55 * * * * * * * * * * * * * In the remainder of this chapter, we’ll cover a few basic rules of algebra that can be used to help us find solutions of equations. We’ll learn when equations are equivalent — either by addition, multiplication, or invertible functions — and we’ll see that equivalent equations have the same solutions. That will allow us to find solutions of equations by creating a sequence of equivalent equations that terminates in an equation that’s easy for us to solve. Equations equivalent by addition In the definition below, D is a set of real numbers. That is, D ⊆ R. The equation f (x) + h(x) = g(x) with domain D is equivalent to the equation f (x) = g(x) − h(x) with domain D Examples. • The equation x2 + 3x = ex with domain R is equivalent to the equation x2 = ex − 3x with domain R. Here we are just using the rule above with f (x) = x2 , h(x) = 3x, and g(x) = ex . • The domains of two equivalent equations must be the same. The equation x2 + 3x = ex with the restricted domain (0, ∞) is equivalent to the equation x2 = ex − 3x with the restricted domain (0, ∞). • To repeat, the domains of two equivalent equations are equal. The equation x2 + 3x = ex with domain [−3, 10] is equivalent to the equation x2 = ex − 3x with domain [−3, 10]. Compare this example to the previous two examples. Equivalent equations have the same domain. • x2 −loge (x) = 0 is equivalent to x2 = loge (x). To see this, just subtract − loge (x) (or add loge (x)) to both sides of the equation x2 −loge (x) = 0. Both 56 equations in this example have an implied domain of (0, ∞), because we can only take the logarithm of a positive number. • Whenever you add or subtract a function from both sides of an equation, you’ll obtain an equivalent equation. Thus, 7x3 − 5 = x2 + 2x is equivalent to 7x3 − x2 − 2x − 5 = 0. Just subtract x2 + 2x from both sides of 7x3 − 5 = x2 + 2x. √ √ • 3x = 2x2 is √ equivalent to 3x + x − 2 = 2x2 + x − 2. Just add x − 2 to both sides of 3x = 2x2 . The importance of equivalent equations is that they have the same sets of solutions. Equivalent equations have the same sets of solutions. Examples. • The equations x2 = −x + 5 and x2 + x − 5 = 0 are equivalent. To get the second equation from the first, just add the function x − 5. Because these two equations are equivalent, they have the same set of solutions. We know how to find the solutions of x2 +x−5 = 0. These √ are just 1 2 the roots 21) and √ of the quadratic polynomial x + x − 5, which are 2 (−1 − 1 2 words, the set of solutions of the equation x +x−5 = 0 2 (−1+ 21). In other √ √ is the set { 12 (−1 − 21), 12 (−1 + 21)}. Therefore, the set of solutions of √ √ x2 = −x + 5 is also { 12 (−1 − 21), 12 (−1 + 21)}. • To find the set of solutions of x2 + x − 4 = x2 , just subtract x2 from both sides of the equation. We’ll be left with the equivalent equation x − 4 = 0. The set of solutions of this latter equation is {4}. Because the two equations in this example are equivalent, the set of solutions of x2 +x−4 = x2 is also {4}. √ • To find the solutions of x − x = 5, we can add the function x to √ both sides of the equation and we’d be left with √ the equivalent equation x = x + 5.√We saw earlier in this chapter that x = x + 5 has no solutions. Therefore, x − x = 5 has no solutions. 57 * * * * * * * * * * * * * Equations equivalent by multiplication We say that the number α ∈ R is a zero of the function h(x) if h(α) = 0. Examples. • 2 is a zero of h(x) = x2 − 4 because h(2) = 22 − 4 = 0. The number −2 is also a zero of h(x) = x2 − 4. • 3 is a zero of h(x) = x − 3 since h(3) = 3 − 3 = 0. • In the previous two examples, h(x) was a polynomial. If h(x) is a polynomial then we usually call a zero of h(x) a root of h(x) instead. It’s just a different name for the same thing. √ √ √ • 5 is a zero of x − 5 because 5 − 5 = 0 = 0. • 1 is a zero of loge (x). • 4 is not a zero of 2x − 5 because 2(4) − 5 = 8 − 5 = 3 6= 0. We say that h(x) has no zeros in a set D ⊆ R if none of the numbers in D is a zero for h(x). Examples. • The linear polynomial h(x) = x − 3 has no zeros in [5, ∞). That’s because if α ∈ [5, ∞), then α ≥ 5. Therefore, h(α) = α − 3 ≥ 5 − 3 = 2, and if h(α) ≥ 2, then h(α) 6= 0. That is, α is not a zero of h(x) if α ∈ [5, ∞). The function h(x) = x − 3 does have a zero. It’s 3. But h(x) has no zeros in [5, ∞). (co 1/” 7 58 • The function h(x) = x2 − 4 has two zeros, 2 and −2. Neither of these zeros are in the set [−1, 1], so h(x) = x2 − 4 has no zeros in [−1, 1]. (co (co 1/” 1/” 7 7 / 1/” • 4 − 2x has exactly one zero, the number 2. Because 2 is the only zero, 4 − 2x has no zeros in R − {2}. (co (co 2 7 1/” 7 // (oo’o) • ex has no zeros. It has no zeros in R. / 22/ (oo’o) (oo’o) • x is a function. It’s the identity function. It only has one zero, the number 0 itself. Thus, x has a zero in R, but it has no zeros in R − {0}. Neither does it have zeros in (0, ∞), for example. 2 (oo’o) 59 In the definition below, D is a subset of real numbers. The equation h(x)f (x) = g(x) with domain D is equivalent to the equation f (x) = g(x) h(x) with domain D as long as h(x) has no zeros in D. The definition above tells us that we can multiply or divide both sides of an equation by a function—as long as the function has no zeros in the domain— and obtain an equivalent equation. Examples. • If h(x) is a constant function, and not the constant 0, then it has no zeros. Therefore, we can always divide both sides of an equation by h(x) to x4 +1 4 obtain an equivalent equation. Thus, 3x = x + 1 is equivalent to x = 3 . √ √ The equation 2 x = 8 is equivalent to x = 4. • If we multiply both sides of an equation by a constant that’s not zero then we’ll also obtain an equivalent equation. x4 = log5 (x) is equivalent to √ √ x = 4 log5 (x). 13 x = 4 is equivalent to x = 12. • The equation x loge (x) = x has a domain of (0, ∞). Because the function x has no zeros in (0, ∞), we can divide both sides of the equation x loge (x) = x by x to obtain the equivalent equation loge (x) = 1. • This is an example of something that can’t be done. If you have the equation x2 = x, then you might be inclined to divide by x, similar to the previous example. We can’t do that here though. The implied domain of the equation x2 = x is R, and x has a zero in R. It has the number 0 as its zero. The two equations x2 = x and x = 1 (which is what we would get by dividing x2 = x by x) are very different equations. The former has two solutions, 0 and 1, while the latter has a single solution, 1. 60 Equivalent equations have the same solutions. Examples. √ √ • The equation 2 x = 8 √ is equivalent by multiplication to x = 4. √ We know the set of solutions of x = 4 is {2}, so 2 x = 8 also has 2 as its only solution. • The equation x loge (x) = x is equivalent to loge (x) = 1. The only solution of loge (x) = 1 is the number e. Therefore, e is the only solution of the equation x loge (x) = x. * * * * * * * * * * * * * Equations equivalent by invertible function Once again, in the definition below D ⊆ R. The equation h(f (x)) = g(x) with domain D is equivalent to the equation f (x) = h−1 (g(x)) with domain D The above definition says that we can erase a function, h, on one side of an equation if we apply its inverse, h−1 to the other side of the equation. Both equations have to have the same domain. Equivalent equations always have the same domain. 61 Examples. • loge (x) = 5 has an implied domain of (0, ∞), because we can only take the logarithm of a positive number. We could erase logarithm base e on the left side of loge (x) = 5 by applying its inverse, exponential base e to the right side. We’d be left with the equation x = e5 with the same domain that we started with, the set (0, ∞). These two equations, loge (x) = 5 and x = e5 , with the same domain, (0, ∞), are equivalent. Equivalent equations always have the same domain. √ √ • The functions 3 x and x3 are inverse functions. Therefore, 3 x − 2 = 7 is equivalent to x − 2 = 73 . (Both equations have a domain of R because we can cube or cube-root any number.) We erased the cube root from the left √ 3 side of x − 2 = 7 by applying its inverse to the right side. Again, what’s important about equivalent equations is the following fact: Equivalent equations have the same solutions Example. • We’ll continue from the previous example. If we have the equation x − 2 = 7, then we know it’s equivalent to the equation x − 2 = 73 . Now 73 = 343, so the previous equation is x − 2 = 343. The only solution of this equation is√x = 343 + 2 = 345. Because 3 x − 2 = 7 and x − 2 = 343 are equivalent, they have the same √ 3 solutions. Therefore, 345 is the only solution of x − 2 = 7. √ 3 * * * * * * * * * * * * * Finding solutions of equations To find the solutions of an equation f (x) = g(x), first write down its domain D. Next, find a sequence of equivalent equations, either equivalent by addition, or by multiplication, or by invertible function. The goal is to have the last equation in this sequence have a set of solutions that is easy to 62 find. It might be really easy, such as in the equation x = 2, or it might be somewhat easy to find, as in the equation x2 +2x−5 = 0, where you can apply the quadratic formula to the quadratic polynomial x2 +2x−5. Whatever easy equation you are left with, find its set of solutions S, but remember to only include in the set S those solutions that are in the domain D of your original equation. Because equivalent equations have the same set of solutions, S will be the set of solutions of your original equation f (x) = g(x). Examples. 2 • Let’s find the solutions of the equation ex +x−6 = 1 where the domain of the equation is the set D = (0, ∞). That means we are only interested in 2 those solution of ex +x−6 = 1 that are positive. 2 Using loge , the inverse function of exponential base e, the equation ex +x−6 = 1 is equivalent to x2 +x−6 = loge (1), which is the equation x2 +x−6 = 0. To find the solutions of this quadratic equation, we use the quadratic formula to find the roots of the quadratic polynomial x2 + x − 6. In doing so, we would find that the solutions of x2 + x − 6 = 0 are −3 and 2. However, the domain of our original equation is (0, ∞), and −3 ∈ / (0, ∞). Therefore, the only relevant solution for us is 2, because 2 ∈ (0, ∞). That means that 2 S = {2} is the set of solutions for our original equation ex +x−6 = 1 with a 2 + xof (0, x 6 = 0 are —3 and 2. However, the domain of our original equation domain ∞). is (0, cc), and —3 (0, cc). Therefore, the only relevant solution for us is 2, because 2 e (0, cc). That means that S = {2} is the set of solutions for our _6 = 1 with a domain of (0, cc). 2 original equation ex — d\ e C— - 1 -3 2. Let’s find the solutions of x 1 = 1 If we aren’t told what the domain of the equation is, then we 63have to find its implied domain. For this equation, the implied domain is R {0}, because we can’t divide by 0. That means that we should only search for solutions of the equation that aren’t zero. We can multiply by the function x to obtain the equivalent equation x —x = 2 . — — 11 - -3 -3 2.2. 1 • Let’s find thethe solutions of of x xx 1 1= wewe aren’t = = 1If IfIf find .. Let’s aren’t solutions told what the x1 .1 find the Let’s we aren’ttold solutions of 1 toldwhat whatthe . If we aren’t told what thethe • Let’s find the solutions of x − 1 = x domain of the equation is, then we have to find its implied domain. For this domain the equation is, then of implied we have to find its domain. For this domain equation then of have implied to find its domain.For Forthis this domain ofthe thethe equation is, is, then wewe have to find itswe implied domain. equation, implied domain is R {0}, because can’t divide by 0. That equation, the implied domain isis RR {0}, because we can’t divide by 0.0. 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They are also the only solutions of x 1 = . equation, also xx 11 =x= They the of only solutions xx 11 == x equation, They are also the of only solutions 1 1 are original equation, x − 1 = . They are the only solutions of x − 1 = . — — — — — — — — — — — . . — . . — — — — x x /x-1 /x-1 p 3 •• The The equation equation 5x 5x 22 == 55has hasan animplied implieddomain domainofofR.R. We Wecan can = 53 √ 3 obtain •the The equation domain equivalent 2by= equation invertible implied5x5x R. WeNow —2 2 = of 5 61has anfunction can by 35x obtain•the equivalent equation invertible function 5 =R. =125. 125. Now The equation 5x − 2 = 5 has an implied domain We can = of 53 127 obtain we the equivalent can 22totoobtain equation invertible add 5x —2x x== = .125. Now 5x by and divide by 5 5totohave we canthe addequivalent obtain 5x==127, 127, and divide by have 3 function obtain equation 5x − 2 = 5 = 125 by erasing the5127cube-root we can = 127,ofand add 2127toisisobtain 5x divide by function, to have since xsince 5function, Of course ininthe domain original our is a areal = Of course the domain of our original real from the left and Now we can add 25 toisobtain 5 applying the cube to the right. p 3 Of course is in the domain original function, 127 of our127 since a real number. Therefore, the set of solutions is the of /5x set number. solutions set { is5 }. 5x = 127, Therefore, and divide the by 5settoofhave x = 5of. 5x 2 2==5 5 isthe number. Therefore, the set of solutions of /5x 2 = 5 is the set127 Of course 127 original function, since 5 is a real 5 is in the domain of our √ number. Therefore, the only solution of 3 5x − 2 = 5 is 127 5 . — — — — • The equation 5x 2 = 5 has an implied domain of R. We can obtain the equivalent equation by invertible function 5x —2 = 53 = 125. Now we can add 2 to obtain 5x = 127, and divide by 5 to have x = Of course is in the domain of our original function, since is a real 1I1in; (wH1 number. Therefore, the set of solutions of /5x 2 = 5 is the set 127 1I1in; (wH1 Dornon 127 Dornon — — 64 ••The has Theequation equation3x 3x 22==0,0, hasaasolution. solution.It’s It’s 23 .However, However,ififweweare are 1I1in; (wH1 told Thethe equation solutions 3xofof3x 23x= solution. It’sofof(—oo, if we 0,2=has awith —2127 are there a adomain toldtoto•find find the solutions =0 0with domain (However, 1,0),0),then then there Dornon — — . . 127 Dornon • The equation 3x − 2 = 0, has a solution. It’s 23 . However, if we are told to find the solutions of 3x − 2 = 0 with a domain of (−∞, 0), then there 2 are no• solutions. The3x set of2 solutions is ∅. That’s because 3 isn’t a The equation However, if negative = 0, has a solution. It’s we are number. told to find the solutions of 3x —2 = 0 with a domain of (—oo, 0), then there . — are no solutions. The set of solutions is 0. That’s because number. Doa’tr 1 62 65 isn’t a negative Exercises For #1-13, match the numbered equations on the left to their lettered implied domains on the right. 1.) loge (x) − loge (x) = 4 A.) R 2.) x2 − 45 = x7 − 3x − 6 B.) (0, ∞) 2 3.) e−7x √ √x x 4.) −3x = x3 C.) [0, ∞) = 3x − 2 D.) [2, ∞) 5.) loge (x − 5) = 23x9 √ 171 6.) 3x − 5 = 2x + 3 2x2 −7x+3 x−5 7.) √ 3 G.) R − {5} √ x−2 x − 7 = 3x + 4 10.) x5 + 11.) F.) (5, ∞) = 3x + 4 8.) loge (x + 7) = 9.) E.) [0, ∞) − {2} √ 20 7x+3 x2 −4 = √ x x − 2 = 3x + 6 12.) 3x − 4x6 = x3 − 13x + 56 13.) √ x = 3x2 − 5 66 For #14-19, match the numbered equations on the left to their lettered sets of solutions on the right. 3 14.) 4x − 3 = 0 A.) 15.) 2x − 6 = 0 √ √ B.) { −1 − 2 2 , −1 + 2 2 } 16.) x + 4 = 0 C.) ∅ 17.) x2 − 2x + 3 = 0 D.) {−4} 18.) 2x2 − 12x + 18 = 0 E.) {3} 4 19.) −x2 − 2x + 7 = 0 For #20-24, match the numbered equations on the left to the lettered equations on the right that they are equivalent to by addition. √ 20.) x2 + x = 3 A.) 5 = 3x + 21.) loge (x) + 2x − 3 = 0 B.) 2x2 − 7x + 3 = 0 22.) 5 − √ x+2 C.) x2 + x − 3 = 0 x = 3x + 2 23.) 3x2 − 4x + 5 = x2 + 3x + 2 D.) −x2 − 3x + 3 = 0 24.) −x2 − 4 = 3x − 7 E.) 2x − 3 = − loge (x) 67 If you ever have an equation p(x) = q(x) where both p(x) and q(x) are polynomials, it’s almost always best to begin by writing the equivalent equation p(x) − q(x) = 0, and then to search for solutions of p(x) − q(x) = 0. Use this method to find the solutions of the equations in problems #25-28. 25.) x2 − 3x + 2 = 3x2 + x − 4 26.) x3 + 3x − 2 = x3 + x2 + x + 1 27.) x2 − 2x + 3 = x2 − x + 4 28.) x − 2 = 3x2 − 5x + 1 For #29-35, match the numbered functions on the left with their lettered set of zeros on the right. 29.) x − 7 A.) {−1, 1} 30.) x + 5 B.) {0} 31.) x2 − 1 C.) {−5} 32.) loge (x) D.) {1} 33.) √ E.) ∅ x 34.) x2 + 1 F.) {7} 35.) x 68 For each equation given in #36-40, decide whether dividing by the function x on both sides of the equation would result in an equivalent equation. For each of these, you’ll want to check whether x has zeros in the implied domain of the given equation. 36.) 3x2 − 5 = 2x + 8 37.) loge (x) = 3x − 5 38.) 39.) 1 x−7 √ = 3x −x = 4x3 − 5x 40.) x2 (x − 7) = x(x − 7)2 For each equation given in #41-45, decide whether dividing by the function x − 7 on both sides of the equation would result in an equivalent equation. For each of these, you’ll want to check whether x − 7 has zeros in the implied domain of the given equation. 41.) 3x2 − 5 = 2x + 8 42.) loge (x) = 3x − 5 43.) 44.) 1 x−7 √ = 3x −x = 4x3 − 5x 45.) x2 (x − 7) = x(x − 7)2 For #46-48, match the numbered equations on the left to the lettered equations on the right that they are equivalent to by multiplication. 46.) x + x = 3 A.) √ 5− x x+2 47.) loge (x) = 2 B.) 1 x 2 48.) 5 − √ = 3x+2 x+2 loge (x) = 2 x C.) ex (x2 + x) = 3ex x = 3x + 2 69 For #49-52, match the numbered equations on the left to the lettered equations on the right that they are equivalent to by invertible function. 2 49.) e3x+5 = e2x−3 A.) 3x + 5 = x 3 50.) loge (x2 + 1) = 2 B.) x2 − 3x + 1 = 8x3 51.) (3x + 5)3 = x2 C.) 3x + 5 = 2x − 3 52.) √ 3 D.) x2 + 1 = e2 x2 − 3x + 1 = 2x Find the set of solutions of each of the equations given in #53-61. 53.) x2 − 3 = x2 + x 54.) loge (x2 ) = 2 where x ∈ [0, ∞) 55.) 4x2 + 2x + 3 = 3x2 + 2x + 2 56.) √ 3 4x + 2 = 4 57.) 3x2 − 6 = x2 − 4x with x > 10 58.) e3x+3 = 1 59.) x5 (x2 + 3x − 2) = 0 where x < 0 60.) (log2 (2x + 1))3 = 8 61.) 455 x2 −9 = 5 where x < 5 70