Introduction to Optimization Problems

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AP Calculus BC › Introduction to Optimization Problems

Questions 1 - 10
1

A rectangle is inscribed under $y=16-x^2$ above the $x$-axis with vertical sides; what should be maximized?​

The $y$-intercept of the parabola, $16$

The parabola’s area between $-4$ and $4$

The rectangle’s perimeter $P(x)=4x+2(16-x^2)$

The $x$-coordinate of the right corner

The rectangle’s area $A(x)=2x(16-x^2)$

Explanation

This problem involves inscribing a rectangle under a parabola, requiring us to identify the optimization objective. With the parabola y = 16 - x² and rectangle corners at (±x, 0) and (±x, 16-x²), the rectangle has width 2x and height 16-x². The area is A(x) = 2x(16-x²), which should be maximized to find the largest possible inscribed rectangle. The perimeter (choice D) would give P = 4x + 2(16-x²), but maximizing perimeter doesn't necessarily give the largest area. For inscribed figure problems, maximize area unless specifically asked for another quantity.

2

A rectangular sheet is folded to form an open-top box by cutting out squares of side $x$ from corners. What quantity is maximized?

The perimeter of the original sheet as a function of $x$

The area of the removed squares as a function of the final volume

The volume of the box as a function of cut size $x$

The height of the box as a function of the sheet’s area

The surface area of the box as a function of cut size $x$

Explanation

This problem involves maximizing the volume of a box created by cutting and folding a flat sheet. When squares of side x are cut from each corner of a rectangular sheet and the sides are folded up, the resulting box has height x. If the original sheet has dimensions L × W, the box has base dimensions (L - 2x) × (W - 2x) and volume V(x) = x(L - 2x)(W - 2x). The domain is 0 < x < min(L/2, W/2). Choice B incorrectly focuses on surface area, but we want maximum storage capacity. The modeling approach is to express the three-dimensional volume in terms of the cut parameter x, accounting for how the cuts affect each dimension of the resulting box.

3

A Norman window consists of a rectangle topped by a semicircle; the perimeter is 10 m. What is the correct setup?

Maximize $A=wh$ subject to $2w + 2h = 10$

Minimize $P=w + 2h$ subject to $wh + \frac{1}{2} \pi \left( \frac{w}{2} \right)^2 = 10$

Minimize $A=wh + \frac{1}{2} \pi \left( \frac{w}{2} \right)^2$ subject to $w + 2h + \pi \frac{w}{2} = 10$

Maximize $A=wh + \frac{1}{2} \pi \left( \frac{w}{2} \right)^2$ subject to $w + 2h + \pi \frac{w}{2} = 10$

Maximize $A=\pi \left( \frac{w}{2} \right)^2$ subject to $\pi w = 10$

Explanation

This problem requires maximizing the area of a Norman window subject to a perimeter constraint. A Norman window consists of a rectangle of width $w$ and height $h$ topped by a semicircle of diameter $w$. The total area is $A = wh + \frac{1}{2} \pi \left( \frac{w}{2} \right)^2 = wh + \frac{\pi w^2}{8}$. The perimeter constraint includes the rectangle's base and two sides plus the semicircular arc: $w + 2h + \pi \frac{w}{2} = 10$. Choice C incorrectly treats it as a simple rectangle, ignoring the semicircular top. The modeling strategy is to carefully account for all geometric components in both the objective function and constraint equation.

4

A wire of length 24 cm is cut into two pieces to form a square and an equilateral triangle. What should be minimized?

The total perimeter as a function of the cut length

The area of the square as a function of the triangle’s side length

The side length of the square as a function of the triangle’s perimeter

The total area of the square and triangle as a function of the cut length

The triangle’s perimeter as a function of the square’s area

Explanation

This problem requires minimizing the total area of two shapes formed from a single wire of fixed length. Let x be the length of wire used for the square, so (24 - x) is used for the equilateral triangle. The square has side length x/4 and area (x/4)² = x²/16. The triangle has perimeter (24 - x) and side length (24 - x)/3, giving area (√3/4)((24 - x)/3)². The total area is A(x) = x²/16 + (√3/36)(24 - x)². Choice A incorrectly focuses on perimeter, but total perimeter is fixed at 24 cm. The optimization approach is to express the combined objective (total area) as a function of how the constraint resource (wire length) is allocated between competing uses.

5

A box with square base has volume $108\text{ in}^3$. If surface area is to be minimized, what is the objective function?

Minimize $V(x)=x^2h$ subject to $2x^2+4xh=108$

Minimize $S(x)=2x^2+4xh$ subject to $x^2h=108$

Maximize $V(x)=x^2h$ subject to $2x^2+4xh=108$

Minimize $S(x)=x^2h$ subject to $2x^2+4xh=108$

Maximize $S(x)=2x^2+4xh$ subject to $x^2h=108$

Explanation

This problem involves minimizing surface area for a box with fixed volume constraint. Given that the box has a square base with side x and height h, the volume constraint is x²h = 108. The surface area consists of the base (x²), top (x²), and four sides (each xh), giving total surface area S = 2x² + 4xh. Since we want to minimize material usage, we minimize surface area as a function of x using the volume constraint to eliminate h. Choice B incorrectly suggests maximizing surface area, which would use the most material rather than least. The fundamental approach is to express the objective function in terms of a single variable using the constraint equation.

6

A company’s demand is $p=80-2x$ dollars per unit. Revenue is $R=xp$. What should be maximized to find best sales level?

Maximize price $p(x)=80-2x$

Maximize demand $x$ subject to $p=80$

Minimize price $p(x)=80-2x$

Maximize revenue $R(x)=x(80-2x)$

Minimize revenue $R(x)=x(80-2x)$

Explanation

This problem involves maximizing revenue using a linear demand function. The demand relationship is p = 80 - 2x, where p is price and x is quantity. Revenue is R = xp = x(80 - 2x) = 80x - 2x². This is a quadratic function in x, and we maximize it to find the optimal sales level. The maximum occurs at x = 80/(2·2) = 20, giving maximum revenue R = 80(20) - 2(20)² = 1600 - 800 = 800. Choice B incorrectly minimizes revenue, which would minimize profit. The fundamental approach is to substitute the demand function into the revenue equation and optimize the resulting quadratic function.

7

A rectangle is formed with one side on the $x$-axis and two vertices on $y=4x-x^2$. What quantity should be maximized?

The $x$-intercepts as a function of the rectangle’s area

The rectangle’s area as a function of the rightmost $x$-coordinate

The rectangle’s perimeter as a function of the parabola’s vertex

The rectangle’s height as a function of its perimeter

The parabola’s maximum value as a function of the rectangle’s width

Explanation

This problem involves maximizing the area of a rectangle with vertices constrained by a parabola and the x-axis. The rectangle has one side along the x-axis and two vertices on the parabola y = 4x - x². If the rectangle extends from x = 0 to x = a, then its width is a and height is 4a - a², giving area A(a) = a(4a - a²) = 4a² - a³. The domain is 0 ≤ a ≤ 4 since the height must be non-negative. Choice B incorrectly focuses on perimeter, but we want maximum enclosed area. The modeling strategy is to parameterize the rectangle using one coordinate (the rightmost x-coordinate), then use the parabola equation to determine the height and express area as a single-variable function.

8

A right circular cylinder must hold $500\text{ cm}^3$. To minimize material, which quantity should be minimized?

The lateral area $2\pi rh$ as a function of $h$

The height $h$ as a function of $r$ with $S=500$

The total surface area $S=2\pi r^2+2\pi rh$ as a function of $r$ with $\pi r^2h=500$

The radius $r$ as a function of $h$ with $S=500$

The volume $V=\pi r^2h$ as a function of $r$

Explanation

This problem involves minimizing the material needed (surface area) for a cylinder with fixed volume constraint. The cylinder must hold 500 cm³, so the volume constraint is πr²h = 500. The total surface area includes the base (πr²), top (πr²), and lateral surface (2πrh), giving S = 2πr² + 2πrh. To minimize material usage, we minimize this surface area as a function of r, using the volume constraint to express h in terms of r. Choice A incorrectly suggests maximizing volume, but volume is fixed by the problem requirement. The optimization approach is to minimize cost-related quantities (here, material surface area) subject to performance constraints (required volume).

9

A rectangle has vertices on the coordinate axes and the line $3x+2y=12$ in the first quadrant. What should be maximized?

Minimize $A=xy$ with constraint $3x+2y=12$

Maximize $A=xy$ with constraint $3x+2y=12$

Minimize $P=2x+2y$ with constraint $xy=12$

Maximize $3x+2y$ with constraint $xy=12$

Maximize $P=2x+2y$ with constraint $xy=12$

Explanation

This problem requires maximizing the area of a rectangle with vertices on coordinate axes and a line. The rectangle has vertices at (0,0), (x,0), (0,y), and (x,y) where the point (x,y) lies on the line 3x + 2y = 12 in the first quadrant. The rectangle's area is A = xy, and we maximize this subject to the line constraint. Using the constraint y = (12 - 3x)/2, we get A(x) = x(12 - 3x)/2 = (12x - 3x²)/2. This is maximized when x = 2, giving y = 3 and maximum area 6. Choice B incorrectly minimizes area, but we want maximum enclosed space. The fundamental approach is to use the line constraint to express area as a function of one coordinate, then optimize that function.

10

A rectangular sign has width $w$ and height $h$ with diagonal fixed at 10. To maximize area, what is the correct constraint?

Use $w^2+h^2=10$ and maximize $A=wh$

Use $w^2+h^2=100$ and maximize $A=wh$

Use $2w+2h=10$ and maximize $A=wh$

Use $w+h=10$ and maximize $A=wh$

Use $wh=10$ and minimize $w^2+h^2$

Explanation

This problem involves maximizing the area of a rectangle with a diagonal length constraint. If the rectangle has width w and height h with diagonal fixed at 10, then the constraint is w² + h² = 100 (since diagonal² = w² + h²). We want to maximize the area A = wh subject to this constraint. Choice B incorrectly uses w + h = 10, which would be a perimeter constraint, not a diagonal constraint. The diagonal of a rectangle with sides w and h is √(w² + h²), so fixing the diagonal at 10 gives w² + h² = 100. The approach is to correctly translate geometric constraints (like fixed diagonal length) into algebraic constraint equations.

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