Solving Optimization Problems

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AP Calculus BC › Solving Optimization Problems

Questions 1 - 10
1

A projectile’s height is $h(t)=-t^2+6t$ for $0 \le t \le 6$; at what time $t$ is height maximized?

$0$

$6$

$2$

$3$

$4$

Explanation

Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. For height $h(t) = -t^2 + 6t$ on $[0, 6]$, set $h'(t) = -2t + 6 = 0$, giving $t = 3$. Evaluate $h(3) = 9$ compared to $h(0) = 0$ and $h(6) = 0$. The negative second derivative confirms a maximum at $t = 3$. A tempting distractor is $t = 6$, but $h(6) = 0$ is the minimum. Always remember to check both critical points and endpoints in closed intervals for optimization problems.

2

For $0\le x\le 6$, which $x$ maximizes $A(x)=x(6-x)$, the area of a rectangle?

$3$

$0$

$6$

$4$

$2$

Explanation

Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. For the area A(x) = x(6 - x) on [0, 6], find critical points by setting A'(x) = 6 - 2x = 0, yielding x = 3. Evaluate at the critical point where A(3) = 9 and at endpoints A(0) = 0 and A(6) = 0. This confirms the maximum at x = 3 since the function is a downward parabola. A tempting distractor is x = 4, but it gives A = 8, which is less than 9. Always remember to check both critical points and endpoints in closed intervals for optimization problems.

3

A company’s profit is $P(x)=-x^2+12x-20$ (thousand dollars); which $x$ maximizes profit?

$12$

$9$

$3$

$6$

$0$

Explanation

Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. To maximize profit $P(x)=-x^2+12x-20$, find the critical point by setting $P'(x)=-2x+12=0$, yielding $x=6$. This critical point is a maximum because the second derivative $P''(x)=-2$ is negative. No specific endpoints are given, but the parabola opens downward, confirming the vertex is the maximum. A tempting distractor is $x=12$, but that gives $P=-20$, which is less than 16 at $x=6$. A transferable strategy for optimization is to express the quantity to optimize as a function of one variable, find critical points by setting the derivative to zero, and use the second derivative test or endpoint evaluation to identify maxima or minima.

4

A rectangle is inscribed in the region under $y=6-x$ in the first quadrant with one corner at origin. What $x$ maximizes area?

$1$

$3$

$0$

$6$

$2$

Explanation

This problem requires solving an optimization problem using calculus to maximize the area of a rectangle under $y=6-x$ in the first quadrant with one corner at the origin. The area $A(x) = x(6 - x)$, and $A'(x) = 6 - 2x$ sets to zero at $x = 3$. Evaluating at $x = 3$ and endpoint $x = 0$ confirms the maximum. The second derivative is negative at $x = 3$. A tempting distractor is $x = 2$, but it fails because $A(2) = 8 < 9$ at $x = 3$. A transferable strategy for optimization is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, and check endpoints if the domain is closed.

5

For $x>0$, minimize $f(x)=\frac{25}{x}+x$. What value of $x$ minimizes $f$?

$ rac{1}{5}$

$25$

$0$

$1$

$5$

Explanation

Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. For $f(x) = \frac{25}{x} + x$ for $x > 0$, compute $f'(x) = -\frac{25}{x^2} + 1 = 0$, giving $x = 5$. The second derivative $f''(x) = \frac{50}{x^3} > 0$ for $x > 0$ confirms a minimum. As $x$ approaches 0 or infinity, $f$ increases. A tempting distractor is $x = 25$, but it gives higher value than at $x = 5$. A transferable optimization-solving strategy is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, evaluate at critical points and endpoints, and use tests to confirm the extremum.

6

For $x>0$, the cost per unit is $C(x)=x+\frac{36}{x}$. What $x$ minimizes $C(x)$?

$12$

$0$

$6$

$3$

$18$

Explanation

Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. For the cost $C(x) = x + \frac{36}{x}$ for $x > 0$, compute the derivative $C'(x) = 1 - \frac{36}{x^2} = 0$, giving $x = 6$. The second derivative $C''(x) = \frac{72}{x^3} > 0$ for $x > 0$ confirms a minimum. As $x$ approaches 0 or infinity, $C$ increases, supporting the minimum at $x = 6$. A tempting distractor is $x = 12$, but it gives higher cost than at $x = 6$. A transferable optimization-solving strategy is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, evaluate at critical points and endpoints, and use tests to confirm the extremum.

7

A rectangle has fixed diagonal $10$; if one side is $x$, what $x$ maximizes its area?

$\sqrt{10}$

$0$

$5$

$\frac{10}{\sqrt{2}}$

$10$

Explanation

This problem requires solving an optimization problem using calculus to maximize the area of a rectangle with fixed diagonal 10 and one side x. The area is A(x) = x $\sqrt{100 - x^2}$, and the derivative A'(x) = $\frac{100 - 2x^2}{\sqrt{100 - x^2}}$ sets to zero when $100 - 2x^2 = 0$, so x = $\frac{10}{\sqrt{2}}$. Assuming domain 0 < x < 10, there are no endpoints, but checking confirms this is the maximum as it forms a square. The second derivative or sign change verifies the maximum. A tempting distractor is $\sqrt{10}$, but it fails because it does not solve the derivative equation and gives a lower area. A transferable strategy for optimization is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, and check endpoints if the domain is closed.

8

A rectangle has perimeter $40$; if its width is $x$, what value of $x$ maximizes its area?

$5$

$20$

$10$

$0$

$15$

Explanation

Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. For the rectangle with fixed perimeter 40 and width x, the length is 20 - x, so the area A(x) = x(20 - x) = 20x - x². To maximize A(x), compute the derivative A'(x) = 20 - 2x and set it to zero, giving the critical point x = 10. The second derivative A''(x) = -2 < 0 confirms a maximum at x = 10; checking endpoints x = 0 and x = 20 yields A = 0, the minimum. A tempting distractor is x = 20, but it results in zero area as the length becomes zero. A transferable optimization-solving strategy is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, evaluate at critical points and endpoints, and use tests to confirm the extremum.

9

A rectangle has fixed area $36$; if one side is $x$, what $x$ minimizes the sum of side lengths $x+\frac{36}{x}$?

$6$

$18$

$9$

$12$

$3$

Explanation

This problem requires solving an optimization problem using calculus to minimize the sum of side lengths x + 36/x for a rectangle with fixed area 36. The sum function is S(x) = x + 36/x, and its derivative S'(x) = 1 - 36/x² sets to zero at x = 6. Assuming x > 0 with no endpoints, this critical point is the minimum as S''(x) > 0 there. Evaluating shows the minimum sum is 12 at x = 6. A tempting distractor is x = 9, but it fails because S(9) = 9 + 4 = 13 > 12. A transferable strategy for optimization is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, and check endpoints if the domain is closed.

10

A line has intercepts $(x,0)$ and $(0,12-x)$ for $0\le x\le12$. What $x$ maximizes triangle area $A=\frac12 x(12-x)$?

$0$

$8$

$6$

$4$

$12$

Explanation

This problem requires solving an optimization problem using calculus to maximize the triangle area A = (1/2) x (12 - x) for 0 ≤ x ≤ 12. The derivative A'(x) = (1/2)(12 - 2x) sets to zero at x = 6. Evaluating at x = 6 and endpoints x = 0 and x = 12 shows A(6) = 18 is the maximum. The second derivative is negative, confirming the maximum. A tempting distractor is x = 8, but it fails because A(8) = (1/2)84 = 16 < 18. A transferable strategy for optimization is to express the quantity as a function of one variable, find critical points by setting the derivative to zero, and check endpoints if the domain is closed.

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