Be Prepared for the Calculus Exam Mark Howell Gonzaga High School, Washington, D.C. Martha Montgomery Fremont City Schools, Fremont, Ohio Practice exam contributors: Benita Albert Oak Ridge High School, Oak Ridge, Tennessee Thomas Dick Oregon State University Joe Milliet St. Mark's School of Texas, Dallas, Texas Skylight Publishing Andover, Massachusetts Copyright © 2005-2011 by Skylight Publishing Chapter 10. Annotated Solutions to Past Free-Response Questions This material is provided to you as a supplement to the book Be Prepared for the AP Calculus Exam. You are not authorized to publish or distribute it in any form without our permission. However, you may print out one copy of this chapter for personal use and for face-to-face teaching for each copy of the Be Prepared book that you own or receive from your school. Skylight Publishing 9 Bartlet Street, Suite 70 Andover, MA 01810 web: e-mail: http://www.skylit.com sales@skylit.com support@skylit.com 2011 AB AP Calculus Free-Response Solutions and Notes Question AB-1 (a) v ( 5.5 ) ­ ≈ −0.453 and a ( 5.5 ) ­ ≈ −1.359 . The velocity is decreasing because v′ ( 5.5 ) = a ( 5.5 ) < 0 . Since v ( 5.5 ) and a ( 5.5 ) have the same sign (both negative), the speed is increasing. 1 6 v ( t ) dt ­ ≈ 1.949 . 6 ∫0 (b) Average velocity = (c) Total distance traveled = ∫ v ( t ) dt ­ ≈ 12.573 . (d) v ( t ) changes sign at ­ t ≈ 5.19552 . Let b = 5.19552 1. 6 0 x ( b ) = x ( 0 ) + ∫ v ( t ) dt = 2 + ∫ v ( t ) dt ­ ≈ 14.135 . b b 0 0 Notes: 1. Store this number in your calculator, and use the stored value in the subsequent calculation. 3 4 FREE-RESPONSE SOLUTIONS ~ 2011 AB Question AB-2 (a) (b) (c) H ′ ( 3.5 ) ≈ H ( 5 ) − H ( 2 ) 52 − 60 8 = = − degrees Celsius/minute. 5−2 3 3 1 10 H ( t ) dt is the average temperature of the tea in degrees Celsius from t = 0 to 10 ∫ 0 t = 10 minutes.1 The trapezoidal approximation gives 1 ⎛ 66 + 60 60 + 52 52 + 44 44 + 43 ⎞ 2 + 3⋅ + 4⋅ + 1⋅ ⎜ 2⋅ ⎟. 10 ⎝ 2 2 2 2 ⎠ By the FTC, ∫ 10 0 ∫ 10 0 H ′ ( t ) dt = H (10 ) − H ( 0 ) = 43 − 66 = −23 degrees Celsius. H ′ ( t ) dt represents the net change in temperature of the tea, in degrees Celsius, over the 10-minute time interval.3 (d) The temperature of biscuits at t = 10 minutes B (10 ) = B ( 0 ) + ∫ B′ ( t ) dt = 100 + ∫ B′ ( t ) dt ­ ≈ 34.183 . The biscuits are 10 10 0 0 43 – 34.183 = 8.817 degrees Celsius cooler than the tea. Notes: 1. 2. 3. It is important to mention the units both for the variable t (minutes) and the average temperature (degrees Celsius). 1 No need to evaluate. For the curious, the value is ≈ ( 529.5 ) = 52.950 . 10 An alternative meaning might be “The tea cooled by 23°C during the 10 minutes,” but that would describe the meaning of the value of the expression, while the problem clearly asks for the meaning of the expression itself. FREE-RESPONSE SOLUTIONS ~ 2011 AB Question AB-3 1 (a) (b) (c) 1⎞ ⎛1⎞ ⎛1⎞ ⎛ f ⎜ ⎟ = 1 . f ′ ( x ) = 24 x 2 so f ′ ⎜ ⎟ = 6 . The tangent line is y − 1 = 6 ⋅ ⎜ x − ⎟ .2 2⎠ ⎝2⎠ ⎝ ⎝2⎠ Area = ∫ 1 2 0 ( sin (π x ) − 8 x ) dx = ⎛⎜⎝ − π1 cos (π x ) − 2 x4 ⎞⎟⎠ Volume = π ∫ 3 1/ 2 0 1 2 0 1 ⎛ 1⎞ 1 1 = 0− −⎜− ⎟ = − . 8 ⎝ π⎠ π 8 ((1 − f ( x )) − (1 − g ( x )) ) dx . 2 2 3 Notes: 1. 2. 3. This is the third consecutive year that an area-volume problem has appeared in the closed calculator part of the free response section. This trend is likely to continue. 1⎞ ⎛ Or y = 1 + 6 ⋅ ⎜ x − ⎟ . Don’t waste time to simplify. 2⎠ ⎝ Use the names of the functions given in the problem to save time and prevent transcription errors. 5 6 FREE-RESPONSE SOLUTIONS ~ 2011 AB Question AB-4 −3 g ( −3) = −6 + ∫ (b) g ′ ( x ) changes sign from positive to negative only once on [−4, 3] , so g has an 0 f ( t ) dt = −6 − 9π 1 . g ′ ( x ) = 2 + f ( x ) , so g ′ ( −3) = 2 + 0 = 2 . 4 (a) absolute maximum when this happens. For −4 ≤ x ≤ 0 , g ′ ( x ) = 2 + f ( x) > 0 . For 0 ≤ x ≤ 3 , g ′ ( x ) = 0 when f ( x ) = −2 . f ( x ) = 3 − 2 x , so f ( x ) = −2 when x = Since g ′ ( x ) > 0 for −4 < x < maximum is at x = (c) 5 . 2 5 5 and g ′ ( x ) < 0 for < x < 3 , the absolute 2 2 5 2 . 2 Only x = 0. g ′ ( x ) = 2 + f ( x ) changes from increasing to decreasing only at x = 0, and g ′ ( x ) never changes from decreasing to increasing. So the graph of g changes from concave up to concave down, at x = 0. (d) f ( 3 ) − f ( −4 ) − 3 + 1 2 = = − . There is no 3 − ( −4 ) 7 7 contradiction with the Mean Value Theorem because the condition that f is differentiable on the interval is not satisfied: f is not differentiable at x = 0. The average rate of change is Notes: 1. 2. The integral from 0 to −3 is negative. Since g ′ ( x ) changes sign from positive to negative only once in the interval, it is easier to use this argument than the candidate test. 7 FREE-RESPONSE SOLUTIONS ~ 2011 AB Question AB-5 (a) dW dt = t= 1 4 1 1 ⋅ (1400 − 300 ) = 44 . The tangent line is y = 1400 + 44t . At t = , 25 4 W ≈ 1411 tons. (b) (c) d 2W 1 ⎛ dW = ⋅⎜ 2 dt 25 ⎝ dt 1 1 ⎞ 1 ⎛ 1 ⎞ (W − 300 ) . For 0 < t < , ⎟ = ⋅ ⎜ ⋅ (W − 300 ) ⎟ = 4 ⎠ 25 ⎝ 25 ⎠ 625 d 2W W ≥ 1400 > 300 , so > 0 . Therefore the tangent line lies under the graph of dt 2 y = W (t ) . Therefore, 1411, the estimate based on the tangent line, underestimates the actual value. 1 1 1 t + C . Substituting W = 1400 at 25 t = 0 we get ln (1100 ) = C . Since W (t ) is increasing, W − 300 > 0 . Solving for W, ∫ W − 300 dW = ∫ 25 dt we get W − 300 = e ⇒ ln W − 300 = t + ln (1100 ) 25 Notes: 1. No need to simplify further. ⇒ W = 300 + e t + ln (1100 ) 25 1 t 25 or W = 300 + 1100e . 8 FREE-RESPONSE SOLUTIONS ~ 2011 AB Question AB-6 (a) f ( x) is continuous when x ≠ 0 . lim+ f ( x ) = 1 and lim− f ( x ) = 1 , so x →0 x →0 lim f ( x ) = 1 = f ( 0 ) . Therefore, f is continuous at x = 0, too. x →0 (b) (c) ⎧−2 cos x, for x < 0 1 ⎛3⎞ 3 . −4e −4 x = −3 ⇒ e −4 x = f ′( x) = ⎨ ⇒ x = − ln ⎜ ⎟ . −4 x 4 ⎝4⎠ 4 ⎩−4e , for x > 0 Average value = 1 1 1 f ( x ) dx = ∫ 1 − ( −1) −1 2 1⎛ ⎜ ( x + 2 cos x ) 2⎜ ⎝ (∫ (1 − 2sin x ) dx + ∫ 0 e−4 x dx −1 0 1 )= 1 ⎛ e −4 1 ⎞ ⎞ 1 ⎛ e −4 x ⎞ ⎞ 1 ⎛ ⎟ 2 1 2 cos 1 − − + − + + = ( )) ⎜ − ⎟ ⎟ . ( ⎜ ⎜ ⎟ −1 ⎝ −4 −4 ⎠ ⎠ ⎝ −4 ⎠ 0 ⎟⎠ 2 ⎝ 0 ( ) Notes: 1. No need to simplify further. If you insist, = 13 1 − cos ( −1) − 4 . 8 8e 2011 BC AP Calculus Free-Response Solutions and Notes Question BC-1 2 (a) Speed = ⎛ dx ⎞ ⎛ dy ⎞ ⎜ ⎟ +⎜ ⎟ ⎝ dt ⎠ ⎝ dt ⎠ ⎛d x d y⎞ G a ( 3) = ⎜ 2 , 2 ⎟ ⎝ dt dt ⎠ 2 2 2 = 132 + sin 2 ( 9 ) ­ ≈ 13.007 . t =3 = ( 4, 6 cos ( 9 ) ) ­ ≈ ( 4, − 5.467 ) . 1 t =3 dy dy = dx dt sin 9 dx . At t = 3, this is ­ ≈ 0.032 . dt 13 (b) The slope is (c) x ( 3) = x ( 0 ) + ∫ (d) Total distance traveled is 3 dx dt = ∫ ( 4t + 1) dt ­ = 21 ; 0 dt 0 3 dy 3 y ( 3) = y ( 0 ) + ∫ dt = −4 + ∫ sin ( t 2 ) dt ­ ≈ −3.226 . 0 dt 0 3 ∫ 0 ( 4t + 1) 3 2 ( + sin ( t 2 ) ) 2 dt ­ ≈ 21.091 Notes: 1. You could use numeric derivatives of dx dy and instead. dt dt Question BC-2 See AB Question 2. 9 10 FREE-RESPONSE SOLUTIONS ~ 2011 BC Question BC-3 1 k 1 + ( 2e 2 x ) dx (a) Perimeter = 1 + k + e 2 k + ∫ (b) Volume = π ∫ (c) dV π dk 1 dV π 1 π e2 . At k = , = ⋅ 4e 2 ⋅ = . = ⋅ 4e 4 k 4 3 3 dt dt 2 dt 4 k 0 (e ) 2x 2 0 2 k π 0 4 dx = π ∫ e 4 x dx = e4 x k 0 = π (e 4 4k − 1) . Notes: 1. This is the third consecutive year that an area-volume problem has appeared in the closed calculator part of the free response section. This trend is likely to continue. Question BC-4 See AB Question 4. Question BC-5 See AB Question 5. FREE-RESPONSE SOLUTIONS ~ 2011 BC 11 Question BC-6 (a) sin x = x − (b) cos x = 1 − x2 x4 x6 + − + ... . 2! 4! 6! ⎛ ⎞ ⎛ x 2 x4 x6 ⎞ x 6 x10 x14 f ( x) = ⎜ x 2 − + − + ... ⎟ + ⎜1 − + − + ... ⎟ = 3! 5! 7! ⎝ ⎠ ⎝ 2! 4! 6! ⎠ 1+ (c) (d) x3 x5 x 7 x 6 x10 x14 + − + ... . sin ( x 2 ) = x 2 − + − + ... 3! 5! 7! 3! 5! 7! f( x2 x4 ⎛ 1 1 ⎞ 6 + − ⎜ + ⎟ x + ... 2 4! ⎝ 3! 6! ⎠ 6) 1 (0) = − ⎛ 1 + 1 ⎞ ⎛ 6! ⎞ ( 6) ⎜ ⎟ ⇒ f ( 0 ) = − ⎜ + 1⎟ = −121 . ⎝ 3! 6! ⎠ ⎝ 3! ⎠ 6! ⎛ 1⎞ on the interval ⎜ 0, ⎟ is less ⎝ 4⎠ The Lagrange error bound is less than From the graph, the maximum value of f ( than 40. 2 5) ( x) 5 40 ⎛ 1 ⎞ 1 1 1 1 ⎛1⎞ ⎛1⎞ . So P4 ⎜ ⎟ − f ⎜ ⎟ < . ⋅⎜ ⎟ = = < 5 5! ⎝ 4 ⎠ 3 ⋅ 4 3 ⋅1024 3000 ⎝4⎠ ⎝ 4 ⎠ 3000 Notes: 1. Leave it at that. 2. ⎛ 1⎞ We chose 40 because undoubtedly f ( 5) ( x ) < 40 on ⎜ 0, ⎟ , 40 is convenient to use ⎝ 4⎠ in calculations, and 40 gives the desired error bound of less than 1 . 35 would 3000 work as well, but the arithmetic would be slightly more difficult. The choice of 30 ⎛1⎞ doesn’t work because f ( 5) ⎜ ⎟ is slightly greater than 30. ⎝4⎠ 2011 AB (Form B) AP Calculus Free-Response Solutions and Notes Question AB-1 (Form B) ∫ 60 S ′ ( t ) dt ­ ≈ 171.813 millimeters. (a) S (0) = 0 . S (60) = S (0) + (b) The average rate of change is (c) The volume at time t, V ( t ) = π r 2 S ( t ) = 100π S ( t ) . The rate of change of the 0 S ( 60 ) − S ( 0 ) 171.813 ≈­ ≈ 2.864 millimeters/day. 60 − 0 60 volume is V ′ ( t ) = 100π S ′ ( t ) . V ′ ( 7 ) = 100π S ′ ( 7 ) ­ ≈ 602.218 mm3/day. (d) D ( 0 ) = M ′ ( 0 ) − S ′ ( 0 ) = 0.825 − 1.5 < 0 and D ( 60 ) = M ′ ( 60 ) − S ′ ( 60 ) = 72.825 − 3.448 > 0 . 1 Since both M ′ ( t ) and S ′ ( t ) are continuous functions, D(t) is continuous. By the Intermediate Value Theorem, D (t ) = 0 for some t between 0 and 60. At that point in time, M ′ ( t ) = S ′ ( t ) . Notes: 1. Note that we are given expressions for S ′(t ) and M (t ) . It might be easier to enter M (t ) into your calculator, then get M ′(0) and M ′(60) numerically. 13 14 FREE-RESPONSE SOLUTIONS ~ 2011 AB (FORM B) Question AB-2 (Form B) (a) 1000 3000 and lim− r ( t ) = . Since lim+ r ( t ) ≠ lim− r ( t ) , t → 5 t →5 t →5 e 8 lim r ( t ) does not exist, so r is not continuous at t = 5. lim r ( t ) = 1000e −0.2⋅5 = t → 5+ t →5 (b) The average value of r ( t ) is 8 1 8 1 ⎛ 5 600t 1 ⎞ = + r t dt dt 1000e −2t dt ⎟ 1­ ≈ (1234.5073 + 829.9146 ) ≈ ( ) ⎜ ∫0 ∫ ∫ 0 5 8 8 8⎝ t +3 ⎠ 258.053 liters per hour. (c) r ′(3) ­ = 50 liters per hour per hour. At t = 3 hours, the drainage rate is increasing at the rate of 50 liters per hour per hour. (d) 12000 − ∫ r ( t ) dt = 9000 . A 0 Notes: 1. Calculating an integral of a piecewise-defined discontinuous function is rare, if not unprecedented, in a free response problem. Nonetheless, integrating such functions is obviously a skill that is tested. FREE-RESPONSE SOLUTIONS ~ 2011 AB (FORM B) 15 Question AB-3 (Form B) ∫ f ( x ) dx + ∫ 4 6 0 4 2 g ( x ) dx = x3/2 3 4 1 16 22 1 + ⋅2⋅2 = + 2 = . 2 3 3 0 (a) Area = (b) Volume = (c) The graph of g has the slope −1 . So we must have 1 1 1 =1 ⇒ x = , y = . f ′( x) = 1 ⇒ 4 2 2 x ∫ 2 0 2 y (6 − y − y 2 ) dy . Notes: 1. It is easier to use geometry — the area of the triangle — for ∫ g ( x ) dx . 6 4 Otherwise, 6 ⎛ 36 ⎞ ⎛ 16 ⎞ x2 ⎞ ⎛ (6 − ) = 6 − x dx x ⎜ ⎟ = ⎜ 36 − ⎟ − ⎜ 24 − ⎟ = 2 . ∫4 2⎠4 ⎝ 2 ⎠ ⎝ 2⎠ ⎝ 6 Question AB-4 (Form B) (a) f has a critical point where f ′ ( x ) = ( 4 − x ) x −3 = 0 ⇒ x = 4 . This is a relative maximum for f since f ′ ( x ) changes sign from positive to negative at x = 4. (b) f ′ ( x ) = 4 x −3 − x −2 ⇒ f ′′( x) = −12 x −4 + 2 x −3 = x −4 (−12 + 2 x) . f ′′( x) < 0 if and only if 0 < x < 6. Therefore, the graph of f is concave down on (0, 6) .1 (c) f ( x) = f (1) + ∫ f ′(t ) dt = 2 + ∫ x 1 x 1 ( 4t −3 − t −2 ) dt = x t −2 t −1 2 1 2 1 = 2 − 2 + − (−2 + 1) = 3 − 2 + . 2 + 4⋅ − −2 −1 1 x x x x Notes: 1. Or (0, 6] . x = 0 must be excluded, because it is not in the domain of f. 16 FREE-RESPONSE SOLUTIONS ~ 2011 AB (FORM B) Question AB-5 (Form B) (a) (b) a ( 5 ) = v′ ( 5 ) ≈ ∫ 60 0 v ( t ) dt is the total distance in meters Ben traveled on his unicycle during the 60 seconds. (c) (d) v (10 ) − v ( 0 ) 2.3 − 2 = = 0.03 meters per second per second. 10 − 0 10 ∫ 60 0 v ( t ) dt ≈ 2.0 ⋅ (10 − 0) + 2.3 ⋅ (40 − 10) + 2.5 ⋅ (60 − 40) meters.1 Yes. The Mean Value Theorem applied to the time interval [40, 60] guarantees a B ( 60 ) − B ( 40 ) 49 − 9 = = 2. time t when v ( t ) = B′ ( t ) = 60 − 40 20 B ( t ) B′ ( t ) 2 L ( t ) L′ ( t ) = 2 B ( t ) B ′ ( t ) ⇒ L′ ( t ) = . At t = 40, L (t ) ( L ( 40 ) ) 2 L′ ( 40 ) = = 144 + ( B ( 40 ) ) 2 ⇒ L ( 40 ) = 144 + 81 = 225 = 15 . So 9 ⋅ 2.5 meters per second. 2 15 Notes: 1. 2. And leave it at that to save time and avoid arithmetic mistakes. (This is equal to 139 meters.) = 1.5 m/sec. Courtesy John Mahoney, ap-calc EDG moderator, http://www.nthf.org/inductee/05mahoney.htm. FREE-RESPONSE SOLUTIONS ~ 2011 AB (FORM B) 17 Question AB-6 (Form B) (a) (b) ∫ 4π − 2π 1 ⎛ x⎞ ⎛ x⎞ f ( x ) dx = ∫ g ( x ) dx − ∫ cos ⎜ ⎟ dx = ⋅ 6π ⋅ 2π − 2sin ⎜ ⎟ − 2π − 2π 2 ⎝2⎠ ⎝2⎠ 4π 4π 4π = 6π 2 . − 2π f has a critical point at x = 0 since g ′ ( 0 ) doesn’t exist and so f ′ ( 0 ) doesn’t exist. 1 ⎛x⎞ For x ≠ 0 , f ′ ( x ) = g ′ ( x ) + ⋅ sin ⎜ ⎟ . On the interval (−2π , 0) , 2 ⎝2⎠ 1 ⎛x⎞ f ′ ( x ) = 1 + sin ⎜ ⎟ , so the equation f ′( x) = 0 has no solutions. On the interval 2 ⎝2⎠ 1 1 ⎛x⎞ ⎛x⎞ (0, 4π ) , f ′ ( x ) = − + sin ⎜ ⎟ . f ′( x) = 0 at x = π , where 1 = sin ⎜ ⎟ . The 2 2 ⎝2⎠ ⎝2⎠ answer is: f has two critical points on (−2π , 4π ) , at x = 0 and at x = π . (c) ⎛ π⎞ h′ ( x ) = 3 g ( 3 x ) ⇒ h′ ⎜ − ⎟ = 3g ( −π ) = 3π . ⎝ 3⎠ 2011 BC (Form B) AP Calculus Free-Response Solutions and Notes Question BC-1 (Form B) See AB Question 1 (Form B). Question BC-2 (Form B) 2 1 π r (θ ) ) dθ ( ∫ 2 π /2 ­ ≈ 47.513 . (a) Area = (b) Solving r (θ ) cos (θ ) = −3 , we get­ θ ≈ 2.017 . At this angle, y = r (θ ) sin (θ ) ­ ≈ 6.272 . (c) y = r (θ ) sin (θ ) = ( 3θ + sin θ ) sin θ ⇒ dy d = dt dθ ( ( 3θ + sin θ ) sin θ ) ddtθ . At 2π dy , ­ ≈ −1.40954 ⋅ 2 ≈ −2.819 . This means that when θ = 2π , the 3 dt 3 y-coordinate of the particle is decreasing at the rate of 2.819 units of length per unit of time. θ= Question BC-3 (Form B) See AB Question 3 (Form B). 19 20 FREE-RESPONSE SOLUTIONS ~ 2011 BC (FORM B) Question BC-4 (Form B) 1 5 1 f ( x ) dx = ( −10 ) = −2 . ∫ 0 5 5 (a) The average value of f on [0, 5] is (b) ∫ (c) Since g ′ ( x ) = f ( x ) and f ( x) < 0 on (0, 5), g is decreasing on [0, 5]. The graph of 10 0 f ( x ) dx = −10 + 27 = 17 ⇒ ∫ ( 3 f ( x ) + 2 ) dx = 3 ⋅17 + 20 = 71 . 10 0 g is concave up where g ′ = f is increasing, that is when 3 < x < 8. g is both concave up and decreasing on (3, 5).1 (d) The arc length is ∫ 20 0 1 + ( h′ ( x ) ) dx = ∫ 20 2 0 2 10 2 ⎛ ⎛ x ⎞⎞ 1 + ⎜ f ′ ⎜ ⎟ ⎟ dx = 2 ∫ 1 + ( f ′ ( u ) ) du , where 0 ⎝ ⎝ 2 ⎠⎠ x 1 u = , du = dx . Therefore, the arc length of the graph of h from 0 to 20 is twice 2 2 the arc length of the graph of f on [0, 10]. The latter is 11 + 18 = 29 , so the arc length of the graph of h from 0 to 20 is equal to 58. Notes: 1. Or [3, 5]. Question BC-5 (Form B) See AB Question 5 (Form B). FREE-RESPONSE SOLUTIONS ~ 2011 BC (FORM B) 21 Question BC-6 (Form B) (a) (b) (c) ln(1 + x3 ) = x3 − 3n x 6 x9 x12 n +1 x + − + ... + ( −1) ⋅ + ... . n 2 3 4 1 1 1 n +1 1 At x = 1, the series is 1 − + − + ... + ( −1) ⋅ + ... . It converges by the 2 3 4 n alternating series convergence test. At x = −1 , the series is 1 1 1 1 −1 − − − − ... − − ... . It diverges, because it is the negative of the harmonic 2 3 4 n series. The interval of convergence is ( −1, 1] . The first four terms of the series for f ′(t ) are f ′(t ) = 3t 2 − 6t 5 9t 8 12t11 + − + ... = 3t 2 − 3t 5 + 3t 8 − 3t11 + ... ⇒ 2 3 4 1 ⎛ 3t 5 3t11 ⎞ 3 3 ′ f (t ) = 3t − 3t + 3t − 3t + ... . g (1) ≈ ∫ ( 3t − 3t ) dt = ⎜ − ⎟ = − . 0 11 ⎠ 0 5 11 ⎝ 5 2 (d) 4 10 16 22 1 4 10 The alternating series error does not exceed the absolute value of first omitted term, 1 3 1 16 ∫ 0 3t dt = 17 < 5 .