Solving Optimization Problems
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AP Calculus BC › Solving Optimization Problems
A projectile’s height is $h(t)=-t^2+6t$ for $0 \le t \le 6$; at what time $t$ is height maximized?
$4$
$6$
$3$
$0$
$2$
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 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.
For $0\le x\le 6$, which $x$ maximizes $A(x)=x(6-x)$, the area of a rectangle?
$3$
$2$
$6$
$0$
$4$
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 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.
For $x>0$, minimize $f(x)=\frac{25}{x}+x$. What value of $x$ minimizes $f$?
$0$
$25$
$1$
$rac{1}{5}$
$5$
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.
A rectangle has fixed area $36$; if one side is $x$, what $x$ minimizes the sum of side lengths $x+\frac{36}{x}$?
$12$
$6$
$18$
$9$
$3$
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.
A right circular cylinder has fixed surface area $S=2\pi r^2+2\pi rh=54\pi$. What radius $r$ maximizes volume?
$2$
$6$
$3$
$ root{18}$
$ root{9}$
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 cylinder with fixed surface area 54π, express h = (27 - r²)/r, volume V(r) = π(27r - r³). To maximize V(r), set V'(r) = π(27 - 3r²) = 0, giving r = 3. The second derivative test confirms a maximum. A tempting distractor is r = 6, but it gives lower volume. 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.
A rectangle has base on the $x$-axis and upper corners on $y=8-2x^2$. If right upper corner is at $x$, what $x$ maximizes area?
$1$
$\sqrt{2}$
$\sqrt{\frac{4}{3}}$
$2$
$0$
Explanation
Solving optimization problems is a key skill in AP Calculus BC, involving finding maximum or minimum values of a function subject to constraints. For the rectangle with base on x-axis and upper corners on $y = 8 - 2x^2$, with right upper at $x$, assuming symmetry, $A(x) = 2x(8 - 2x^2)$. To maximize $A(x)$, set $A'(x) = 16 - 12x^2 = 0$, giving $x = \sqrt{\frac{4}{3}}$. Endpoints give zero area. A tempting distractor is $x = \sqrt{2}$, but it gives lower area. 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.
A triangle has base $x$ and height $12-x$ for $0\le x\le12$. What $x$ maximizes its area?
$8$
$6$
$0$
$4$
$12$
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 triangle with base x and height 12 - x on [0, 12], area A(x) = (1/2)x(12 - x). To maximize A(x), set A'(x) = (1/2)(12 - 2x) = 0, giving x = 6. Endpoints A(0) = 0 and A(12) = 0 confirm the maximum. A tempting distractor is x = 12, but it gives zero area. 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.
A wire of length $24$ is cut into two pieces forming squares; if first piece length is $x$, which $x$ minimizes total area?
$24$
$12$
$16$
$8$
$0$
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 minimize the total area of two squares from a 24-unit wire cut at x, express A(x)=[x² + (24-x)²]/16 and find the critical point by setting A'(x)=(1/4)x - 3=0, yielding x=12. This critical point is a minimum because the second derivative A''(x)=1/4 is positive. Evaluating at endpoints x=0 and x=24 gives A=36, while A(12)=18 is smaller. A tempting distractor is x=0, but that gives a larger area of 36 compared to 18 at x=12. 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.
For $0\le x\le 6$, the profit is $P(x)=x(6-x)^2$; which $x$ gives the maximum profit?
$6$
$2$
$0$
$4$
$3$
Explanation
This problem requires solving an optimization problem to find the production level that maximizes profit. Expanding P(x) = x(6-x)² = x(36 - 12x + x²) = 36x - 12x² + x³. Taking the derivative: P'(x) = 36 - 24x + 3x² = 3(x² - 8x + 12) = 3(x-2)(x-6). Critical points occur at x = 2 and x = 6. Evaluating: P(0) = 0, P(2) = 2(4)² = 32, P(6) = 6(0)² = 0. Therefore, x = 2 gives the maximum profit of 32. Students might choose x = 3 (the midpoint), but this gives P(3) = 3(3)² = 27 < 32. The strategy is to find all critical points and evaluate at both critical points and endpoints to identify the global maximum.
A company’s profit is $P(x)=-x^2+12x-20$ (thousand dollars); which $x$ maximizes profit?
$0$
$3$
$12$
$9$
$6$
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.