Math 172.503 Exam 2 Solutions March 23, 2010 1. E : Use the trigonometric substitution x = Z Z 2. A : 1 ∞ 1 dx = lim b→∞ e2x e 1 −2x tan θ, dx = 2 3 sec2 θ dθ. Then ( 2 tan θ)2 2 √ 3 · sec2 θ dθ 2 3 4 tan θ + 4 Z 4 2 tan θ 2 9 · sec2 θ dθ = 2 sec θ 3 Z 4 = tan2 θ sec θ dθ. 27 x2 √ dx = 9x2 + 4 b Z 2 3 Z b 1 −2x 1 1 1 1 dx = lim − e = lim − 2b + 2 = 0 + 2 = 2 . b→∞ b→∞ 2 2e 2e 2e 2e 1 3. B : Because x4 − 1 = (x2 − 1)(x2 + 1) = (x − 1)(x + 1)(x2 + 1), we see that the denominator has two distinct linear factors and one irreducible quadratic factor. Therefore, the partial fraction decomposition is 1 A B Cx + D = + + 2 . 4 x −1 x−1 x+1 x +1 4. A : Splitting [1, 3] into n = 6 equal subintervals, we find width of each subinterval is the end-points of the subintervals are 3−1 6 5 7 8 4 , x2 = , x3 = 2, x4 = , x5 = , x6 = 3. 3 3 3 3 x0 = 1, x1 = Checking with the formula for the trapezoidal rule, we see that the answer must be A. 5. C : We quickly evaluate the three integrals: ∞ Z Z b h ib 1 dx = lim ln x = lim [ln b − 0] = ∞, b→∞ b→∞ 1 1 1 x Z ∞ Z 0 Z b sin x dx = lim sin x dx + lim sin x dx −∞ 1 dx = lim b→∞ x a→−∞ = lim a→−∞ Z 0 1 b→∞ a h − cos x i0 h 0 + lim − cos x a b→∞ ib 0 = lim [−1 − cos a] + lim [cos b + 1] (neither limit exists), a→−∞ b→∞ Z 1 h √ i1 √ 1 1 √ dx = lim √ dx = lim 2 x = lim [2 − 2 a] = 2. + + + x x a→0 a→0 a a→0 a Thus, I and II both diverge, and III converges. = 23 . Thus Z 6. To find the arc length of the curve, we use the arc length formula L = Z 2 q 3x1/2 2 0 q y0 2 + 1 dx. We use a y = 2x3/2 and [a, b] = [0, 2]. Since y 0 = 3x1/2 , we have L= b Z 2 + 1 dx = √ 9x + 1 dx 0 19 1√ u du (using u = 9x + 1, du = 9 dx) 9 1 19 2 3/2 2 3/2 2 . = u 19 − = 27 27 27 1 Z = 7. (a) For this integral, we first perform long division and find x4 16 dx = x2 − 4 + 2 , +4 x +4 x2 so Z x4 dx = 2 x +4 Z 16 dx x2 + 4 Z x3 1 = − 4x + 16 · 2 sec2 θ dθ 3 4 sec2 θ x3 − 4x + 8θ + C = 3 x3 x = − 4x + 8 tan−1 +C . 3 2 x2 − 4 + (using x = 2 tan θ, dx = 2 sec2 θ dθ) R It is also possible to use the formula x21+1 = tan−1 (x) + C by pulling a 4 out of the denominator of 1 x2 +4 and using an appropriate u-substitution. (b) We factor x3 − x = x(x2 − 1) = x(x − 1)(x + 1). So we make the partial fraction decomposition 6x − 4 A B C = + . 3 x −x x x+1x−1 Adding the right-hand side together we find, 6x − 4 (A + B + C)x2 + (−B + C)x − A = , 3 x −x x3 − x and so we obtain the system of equations A+B+C =0 −B + C = 6 −A = 4. Solving this system, we find A = 4, B = −5, and C = 1. Therefore, Z Z 6x − 4 4 5 1 dx = − + dx = 4 ln |x| − 5 ln |x + 1| + ln |x − 1| + C . x3 − x x x+1 x−1 dy −2 dx . Integrating both sides we find =√ y 1 − x2 Z Z dy dx = −2 √ y 1 − x2 ln |y| = −2 sin−1 (x) + C. 8. We use separation of variables and find −1 Then exponentiating both sides, we find y = Ae−2 sin this equation as a linear first order equation. (x) for a constant A. It is also possible to solve 9. This is a linear first order equation, so we first divide by x2 to put it in the normal form, dy 2 sin(3x) . + y= dx x x2 R 2 x 1 x2 Z The integrating factor v(x) = e y= 1 v(x) Z v(x)Q(x) dx = dx = e2 ln x = x2 . Therefore, with Q(x) = x2 · 1 sin(3x) dx = 2 2 x x sin(3x) x2 , we have Z sin(3x) dx 1 1 C cos(3x) = 2 − cos(3x) + C = − + 2. 2 x 3 3x x Now using the initial condition that y(π/6) = 2, we substitute x = π/6 into this expression to solve 2 for C. We find that C = π18 (recalling that cos(π/2) = 0). Therefore, y=− cos(3x) π2 . + 2 3x 18x2