Conic Sections

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AP Precalculus › Conic Sections

Questions 1 - 10
1

In an orbital ellipse, $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$ where $(h,k)$ is the center. Refer to the equation provided in the passage: which ordered pair is the center of $\frac{(x+3)^2}{16}+\frac{(y-5)^2}{25}=1$?​

$(-3,-5)$

$(3,-5)$

$(-3,5)$

$(3,5)$

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to identify the center of an ellipse from its equation. In the standard form $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$, the center is at $(h,k)$ where the expressions are $(x-h)$ and $(y-k)$. In the passage equation $\frac{(x+3)^2}{16}+\frac{(y-5)^2}{25}=1$, we need to rewrite $(x+3)^2$ as $(x-(-3))^2$ to identify $h=-3$, and $(y-5)^2$ already shows $k=5$. Choice B correctly identifies the center as $(-3,5)$ by recognizing that $(x+3)=(x-(-3))$. Choice A is incorrect because it fails to account for the sign change when $(x+3)$ is rewritten in standard form. To help students: Always rewrite equations in the form $(x-h)$ and $(y-k)$ before identifying the center, and practice with equations that have addition inside the parentheses. Watch for: sign errors when converting from $(x+a)$ to $(x-h)$ form.

2

In astronomy, Kepler described planetary orbits as ellipses. Consider the standard form $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$ with $a>b>0$, where $(h,k)$ is the center. Based on the conic section described, how does increasing $a$ affect the ellipse’s horizontal extent?

It widens left and right, increasing the major axis length

It shrinks the ellipse vertically, decreasing the minor axis

It shifts the center to the right by $a$ units

It makes the ellipse a hyperbola when $a>b$

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to interpret and manipulate ellipse equations. In the standard form of an ellipse $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$ where $a>b>0$, the parameter $a$ represents the semi-major axis length along the horizontal direction, while $b$ represents the semi-minor axis length along the vertical direction. In the passage, the equation demonstrates that increasing parameter $a$ directly increases the horizontal extent of the ellipse, making it wider from left to right. Choice A correctly identifies that increasing $a$ widens the ellipse left and right, increasing the major axis length to $2a$, which aligns with the mathematical definition of an ellipse's semi-major axis. Choice B is incorrect because changing $a$ affects the shape, not the center position $(h,k)$, a common mistake when students confuse shape parameters with translation parameters. To help students: Use visual demonstrations showing how changing $a$ stretches the ellipse horizontally while keeping the center fixed. Encourage students to graph ellipses with different $a$ values using technology to see the direct relationship between $a$ and horizontal width.

3

Elliptical orbits use $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$. Refer to the equation provided in the passage: which equation has center $(-4,0)$ and $a^2=36$, $b^2=16$?​

$\frac{(x+4)^2}{36}+\frac{y^2}{16}=1$

$\frac{(x+4)^2}{6}+\frac{y^2}{4}=1$

$\frac{(x-4)^2}{36}+\frac{y^2}{16}=1$

$\frac{(x+4)^2}{36}-\frac{y^2}{16}=1$

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to construct ellipse equations from given information. We need an ellipse with center $(-4,0)$, so $h=-4$ and $k=0$, with $a^2=36$ and $b^2=16$ as the squared semi-axis lengths. In the passage format $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$, we substitute to get $\frac{(x-(-4))^2}{36}+\frac{(y-0)^2}{16}=1$, which simplifies to $\frac{(x+4)^2}{36}+\frac{y^2}{16}=1$. Choice A correctly shows this equation with $(x+4)^2$ representing $(x-(-4))^2$ and the given values for $a^2$ and $b^2$. Choice C is incorrect because it uses $a=6$ and $b=4$ instead of $a^2=36$ and $b^2=16$ in the denominators. To help students: Always work with the squared form of semi-axis lengths in the standard equation, and practice converting between different representations of the center. Watch for: using $a$ and $b$ instead of $a^2$ and $b^2$, and sign errors with negative center coordinates.

4

Architects design circular domes using $ (x-h)^2+(y-k)^2=r^2 $. Based on the conic section described, which parameter change increases the circle’s size without moving its center?

Increase $h$

Replace $r^2$ with $r$

Increase $r$

Decrease $k$

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to interpret circle equations and their parameters. In the standard form of a circle $(x-h)^2+(y-k)^2=r^2$, the parameter $r$ represents the radius, $(h,k)$ represents the center, and changing $r$ affects only the size while changing $h$ or $k$ affects only the position. In the passage, the equation demonstrates that to increase a circle's size without moving its center, one must increase the radius parameter $r$. Choice A correctly identifies that increasing $r$ makes the circle larger while keeping the center at $(h,k)$ unchanged, which directly follows from the geometric definition of a circle. Choice B is incorrect because increasing $h$ would shift the circle horizontally to the right, a common mistake when students confuse size changes with position changes. To help students: Use concentric circles to demonstrate how different radii create different sized circles with the same center. Emphasize that $(h,k)$ controls position while $r$ controls size, and these are independent properties.

5

Acoustics teams can locate a sound source using hyperbolas: points with a constant difference in distances to two microphones form a hyperbola. A standard form is $$\frac{(x-h)^2}{a^2}-\frac{(y-k)^2}{b^2}=1,$$ where $(h,k)$ is the center and $a,b>0$ scale the branches. Based on the conic section described, identify the conic represented by $\frac{(x+3)^2}{16}-\frac{(y-2)^2}{4}=1$.

Ellipse centered at $(-3,2)$.

Hyperbola centered at $(-3,2)$.

Parabola with vertex at $(-3,2)$.

Circle centered at $(-3,2)$.

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to identify hyperbolas from their standard equations. The equation $\frac{(x+3)^2}{16}-\frac{(y-2)^2}{4}=1$ follows the standard hyperbola form $\frac{(x-h)^2}{a^2}-\frac{(y-k)^2}{b^2}=1$ with a subtraction between the squared terms. The center is at $(h,k) = (-3,2)$ since $(x+3)^2 = (x-(-3))^2$ and $(y-2)^2$ appear in the equation. Choice B correctly identifies this as a hyperbola centered at $(-3,2)$ because the equation has the characteristic subtraction sign between squared terms that defines a hyperbola. Choice A is incorrect because an ellipse would have addition instead of subtraction between the squared terms, a fundamental distinction students must recognize. To help students: Emphasize that the operation (+ or -) between squared terms determines whether it's an ellipse/circle (+) or hyperbola (-). Watch for: students focusing only on the center coordinates while overlooking the critical sign difference.

6

An elliptical orbit is written in standard form as $$\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1,$$ where $(h,k)$ is the center and $a,b>0$ set the radii. In mission planning, changing $h$ and $k$ repositions the orbit without changing its size. Based on the passage, what is the effect of increasing $h$ while holding $a,b,$ and $k$ constant?

The ellipse shifts up with the same radii.

The ellipse becomes wider horizontally.

The ellipse becomes a hyperbola.

The ellipse shifts right with the same radii.

Explanation

This question tests AP Precalculus understanding of conic sections, particularly how center parameters affect ellipse position. In the standard ellipse equation $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$, the parameters $(h,k)$ determine the center location while $a$ and $b$ control the size. Increasing $h$ while keeping other parameters constant shifts the entire ellipse horizontally to the right without changing its shape or size. Choice A correctly identifies that the ellipse shifts right with the same radii because changing $h$ only affects the x-coordinate of the center, translating the entire figure horizontally. Choice B is incorrect because it confuses center translation with size change, mistaking the role of $h$ (position) with the role of $a$ (horizontal radius). To help students: Use transformation sequences showing how changing different parameters affects the ellipse independently. Watch for: mixing up which parameters control position versus size in conic equations.

7

Architects design circular domes using the circle equation $$ (x-h)^2+(y-k)^2=r^2. $$ The center is $(h,k)$ and $r$ is the radius, which determines the dome’s span. Suppose the design keeps $(h,k)$ fixed but changes $r$. Refer to the equation provided in the passage: based on the passage, what is the effect of increasing $r$?

The circle’s center changes to $(r,r)$.

The circle enlarges while keeping the same center.

The circle becomes an ellipse with major axis $r$.

The circle shifts upward by $r$ units.

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to interpret circle equations and their parameters. In the standard circle equation $(x-h)^2+(y-k)^2=r^2$, the parameter $r$ represents the radius, determining the circle's size while $(h,k)$ fixes its center position. The passage describes how architects modify the dome's span by changing $r$ while keeping the center coordinates constant. Choice A correctly identifies that increasing $r$ enlarges the circle while maintaining the same center because $r$ only affects the distance from center to circumference, not the center's location. Choice B is incorrect because it confuses the radius parameter with vertical translation, a common error when students mix up size and position parameters in conic equations. To help students: Use dynamic geometry software to show how changing $r$ affects only the circle's size. Watch for: misconceptions about which parameters control size versus position in standard form equations.

8

In projectile motion, a ball’s path often follows a parabola described by $$y=ax^2+bx+c.$$ The constant $a$ controls how sharply the path curves, while $b$ and $c$ affect tilt and vertical placement. Coaches compare two throws by changing only $a$ and keeping $b$ and $c$ fixed. Based on the conic section described, what is the effect of increasing $|a|$ on the trajectory’s shape?

The parabola becomes wider and less curved.

The parabola becomes narrower and more curved.

The parabola changes into an ellipse.

The parabola’s vertex shifts right by $|a|$ units.

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to interpret parabola equations and their parameters. In the quadratic form $y=ax^2+bx+c$, the coefficient $a$ controls the parabola's curvature and opening direction, with larger $|a|$ values creating sharper curves. The passage explains how coaches compare throws by varying only the parameter $a$ while keeping other parameters constant. Choice B correctly identifies that increasing $|a|$ makes the parabola narrower and more curved because a larger absolute value of $a$ causes the y-values to change more rapidly as x changes, creating a steeper curve. Choice A is incorrect because it reverses the relationship between $|a|$ and curvature, a common misconception when students think larger values always mean wider shapes. To help students: Graph multiple parabolas with different $a$ values on the same axes to visualize the effect. Watch for: confusion about how the magnitude versus sign of $a$ affects the parabola's shape and direction.

9

In astronomy, planetary paths can be approximated by ellipses following Kepler’s Laws. A common model is $$\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1,$$ where $(h,k)$ is the center and $a,b>0$ set the horizontal and vertical radii. Identify the conic section represented by the equation $\frac{(x-2)^2}{25}+\frac{(y+1)^2}{9}=1$.

Circle centered at $(2,-1)$.

Parabola with vertex at $(2,-1)$.

Ellipse centered at $(2,-1)$.

Hyperbola centered at $(2,-1)$.

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to identify conic types from their standard equations. The given equation $\frac{(x-2)^2}{25}+\frac{(y+1)^2}{9}=1$ follows the standard ellipse form $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$ with both squared terms having the same sign (positive) and being added. The center is at $(h,k) = (2,-1)$ since we have $(x-2)^2$ and $(y-(-1))^2 = (y+1)^2$ in the numerators. Choice A correctly identifies this as an ellipse centered at $(2,-1)$ because the equation matches the ellipse standard form with $a^2=25$ and $b^2=9$. Choice D is incorrect because a circle would require $a^2=b^2$, but here $25 \neq 9$, making this an ellipse rather than a circle. To help students: Create a flowchart for identifying conic sections based on equation structure and coefficient relationships. Watch for: confusion between ellipses and circles, which are special cases of ellipses with equal radii.

10

A spacecraft’s orbit is modeled by an ellipse, as in Kepler’s Laws. The standard form is $$\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1.$$ Here, $(h,k)$ is the center, $a$ is the semi-major axis, and $b$ is the semi-minor axis. Engineers adjust $a$ to widen or tighten the orbit while keeping the center fixed. Refer to the equation provided in the passage: how does increasing $a$ affect the ellipse’s horizontal extent?

It decreases the horizontal radius (semi-major axis).

It increases the horizontal radius (semi-major axis).

It changes the ellipse into a hyperbola.

It shifts the center to the right by $a$ units.

Explanation

This question tests AP Precalculus understanding of conic sections, particularly the ability to interpret and manipulate ellipse equations. In the standard form of an ellipse, $\frac{(x-h)^2}{a^2}+\frac{(y-k)^2}{b^2}=1$, the parameter $a$ represents the horizontal semi-axis length, determining how far the ellipse extends horizontally from its center. The passage describes how engineers adjust $a$ to modify the spacecraft's orbital width while maintaining the same center position. Choice A correctly identifies that increasing $a$ increases the horizontal radius because in the ellipse equation, a larger $a$ value in the denominator means the ellipse extends farther along the x-axis. Choice C is incorrect because it confuses the role of $a$ as a scaling parameter with the center coordinates $(h,k)$, a common mistake when students misinterpret the standard form. To help students: Use visual demonstrations showing how changing $a$ stretches or compresses the ellipse horizontally while the center remains fixed. Watch for: confusion between parameters that affect size versus position in conic equations.

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