Derive Equations of Ellipses and Parabolas

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Geometry › Derive Equations of Ellipses and Parabolas

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1

A hyperbola is the set of all points $P(x,y)$ such that the absolute value of the difference of distances from $P$ to two foci is constant. The foci are $F_1(0,-5)$ and $F_2(0,5)$, and for every point on the hyperbola, $|PF_2-PF_1|=6$. Which equation represents the hyperbola?

$\dfrac{y^2}{25}+\dfrac{x^2}{16}=1$

$\dfrac{y^2}{9}-\dfrac{x^2}{16}=1$

$\dfrac{x^2}{9}-\dfrac{y^2}{16}=1$

$\dfrac{(y-5)^2}{9}-\dfrac{x^2}{16}=1$

Explanation

This problem requires deriving a hyperbola equation from its geometric definition. A hyperbola is the set of all points where the absolute difference of distances to two foci is constant. The foci F₁(0,-5) and F₂(0,5) lie on the y-axis with center at origin and c=5, and |PF₂-PF₁|=6 means 2a=6, so a=3. For a hyperbola, c²=a²+b², giving 25=9+b², so b²=16. Since the foci are on the y-axis, this is a vertical hyperbola with equation y²/a²-x²/b²=1, yielding y²/9-x²/16=1. A common mistake is using the horizontal form x²/a²-y²/b²=1, but the foci location determines orientation. Always start by identifying whether foci align vertically or horizontally to choose the correct standard form.

2

An ellipse has foci at $F_1(-1,2)$ and $F_2(7,2)$. For any point $P(x,y)$ on the ellipse, $PF_1+PF_2=12$. Which equation represents the ellipse? (No vertices are given.)

$\dfrac{(x-3)^2}{36}+\dfrac{(y-2)^2}{20}=1$

$\dfrac{(x-3)^2}{36}-\dfrac{(y-2)^2}{20}=1$

$\dfrac{(x-3)^2}{20}+\dfrac{(y-2)^2}{36}=1$

$\dfrac{(x-3)^2}{9}+\dfrac{(y-2)^2}{27}=1$

Explanation

We need to derive the equation of an ellipse from its foci and distance sum. An ellipse is defined as all points P(x,y) where PF₁ + PF₂ equals a constant. The foci F₁(-1,2) and F₂(7,2) have the same y-coordinate, so the major axis is horizontal and the center is at ((−1+7)/2, 2) = (3,2). With PF₁ + PF₂ = 12, we have 2a = 12, so a = 6 and a² = 36. The distance between foci is 2c = 8, so c = 4, and b² = a² - c² = 36 - 16 = 20, giving (x-3)²/36 + (y-2)²/20 = 1. A common error is confusing which denominator is larger; remember that a > b for ellipses. Start by finding the center as the midpoint of the foci.

3

A hyperbola has foci $F_1(4,-1)$ and $F_2(4,7)$. For every point $P$ on the hyperbola, $|PF_2-PF_1|=2$. Which equation represents the hyperbola?

$\dfrac{(y+1)^2}{1}-\dfrac{(x-4)^2}{15}=1$

$\dfrac{(y-3)^2}{1}-\dfrac{(x-4)^2}{15}=1$

$\dfrac{(y-3)^2}{4}-\dfrac{(x-4)^2}{15}=1$

$\dfrac{(x-4)^2}{1}-\dfrac{(y-3)^2}{15}=1$

Explanation

This hyperbola problem requires applying the geometric definition: points where the absolute difference of distances to foci is constant. The foci F₁(4,-1) and F₂(4,7) share x-coordinate 4, making this a vertical hyperbola centered at (4,(-1+7)/2)=(4,3). The focal distance is 2c=8, so c=4, and |PF₂-PF₁|=2 gives 2a=2, so a=1. Using c²=a²+b² for hyperbolas, we get 16=1+b², so b²=15. The vertical hyperbola form is (y-k)²/a²-(x-h)²/b²=1, yielding (y-3)²/1-(x-4)²/15=1. A common error is using a²=4 thinking 2a=2 means a=2, but 2a represents the full difference, so a=1. Always verify that c>a, which confirms valid hyperbola geometry with our c=4>a=1.

4

An ellipse has foci $F_1(2,-1)$ and $F_2(10,-1)$. For every point $P$ on the ellipse, $PF_1+PF_2=14$. Which equation represents the ellipse?

$\dfrac{(x-6)^2}{49}-\dfrac{(y+1)^2}{33}=1$

$\dfrac{(x-6)^2}{49}+\dfrac{(y+1)^2}{33}=1$

$\dfrac{(x-2)^2}{49}+\dfrac{(y+1)^2}{33}=1$

$\dfrac{(x-6)^2}{33}+\dfrac{(y+1)^2}{49}=1$

Explanation

To derive this ellipse equation, we start with the geometric definition: all points where the sum of distances to two foci is constant. The foci F₁(2,-1) and F₂(10,-1) have the same y-coordinate, so they're horizontal with center at ((2+10)/2, -1) = (6,-1). The distance between foci is 2c=8, so c=4, and PF₁+PF₂=14 gives 2a=14, so a=7. Using c²=a²-b², we get 16=49-b², so b²=33. For a horizontal ellipse centered at (h,k), the equation is (x-h)²/a²+(y-k)²/b²=1, giving (x-6)²/49+(y+1)²/33=1. Students often confuse which denominator is a² versus b², but a² is always under the term corresponding to the foci's direction (horizontal here). Verify by checking that the center-to-focus distance equals c=4.

5

A hyperbola has foci $F_1(-1,-6)$ and $F_2(-1,2)$. For every point $P$ on the hyperbola, $|PF_2-PF_1|=4$. Which equation represents the hyperbola?

$\dfrac{(y+2)^2}{16}-\dfrac{(x+1)^2}{12}=1$

$\dfrac{(y+6)^2}{4}-\dfrac{(x+1)^2}{12}=1$

$\dfrac{(y+2)^2}{4}-\dfrac{(x+1)^2}{12}=1$

$\dfrac{(x+1)^2}{4}-\dfrac{(y+2)^2}{12}=1$

Explanation

This hyperbola problem starts with identifying the geometric constraint: the absolute difference of distances to two foci is constant. The foci F₁(-1,-6) and F₂(-1,2) share x-coordinate -1, making this a vertical hyperbola centered at (-1,(-6+2)/2)=(-1,-2). The focal distance is 2c=8, so c=4, and |PF₂-PF₁|=4 gives 2a=4, so a=2. For hyperbolas, c²=a²+b², yielding 16=4+b², so b²=12. The vertical hyperbola form is (y-k)²/a²-(x-h)²/b²=1, giving (y+2)²/4-(x+1)²/12=1. A common error is using the wrong sign pattern or swapping a² and b² positions, but remember that a² goes under the positive term matching the foci orientation. Always verify that c>a for hyperbolas, confirming our focal geometry.

6

An ellipse is defined as the set of points $P(x,y)$ such that $PF_1+PF_2$ is constant. The foci are $F_1(0,-4)$ and $F_2(0,4)$, and $PF_1+PF_2=12$. Which equation represents the ellipse?

$\dfrac{y^2}{16}+\dfrac{x^2}{20}=1$

$\dfrac{y^2}{36}+\dfrac{x^2}{20}=1$

$\dfrac{y^2}{36}-\dfrac{x^2}{20}=1$

$\dfrac{x^2}{36}+\dfrac{y^2}{20}=1$

Explanation

To derive this ellipse equation, we apply the definition: points where the sum of distances to two foci equals a constant. The foci F₁(0,-4) and F₂(0,4) lie on the y-axis with center at origin, and c=4. Given PF₁+PF₂=12, we have 2a=12, so a=6. Using the ellipse relationship c²=a²-b², we get 16=36-b², so b²=20. Since foci are vertical, this is a vertical ellipse with form y²/a²+x²/b²=1, yielding y²/36+x²/20=1. Students often write x²/36+y²/20=1 by habit, but the larger denominator (a²=36) must go under the variable matching the foci direction (y here). To verify, check that any point satisfying the equation maintains the sum of distances equal to 12.

7

A hyperbola has foci $F_1(-6,3)$ and $F_2(2,3)$. For every point $P$ on the hyperbola, $|PF_2-PF_1|=6$. Which equation represents the hyperbola?

$\dfrac{(x+2)^2}{9}-\dfrac{(y-3)^2}{7}=1$

$\dfrac{(x+2)^2}{7}-\dfrac{(y-3)^2}{9}=1$

$\dfrac{(y-3)^2}{9}-\dfrac{(x+2)^2}{7}=1$

$\dfrac{(x-2)^2}{9}-\dfrac{(y-3)^2}{7}=1$

Explanation

This hyperbola derivation begins with the geometric definition: points where the absolute difference of distances to foci is constant. The foci F₁(-6,3) and F₂(2,3) share y-coordinate 3, indicating a horizontal hyperbola centered at ((-6+2)/2,3)=(-2,3). The focal distance is 2c=8, so c=4, and |PF₂-PF₁|=6 gives 2a=6, so a=3. Using c²=a²+b² for hyperbolas, we get 16=9+b², so b²=7. The horizontal hyperbola form is (x-h)²/a²-(y-k)²/b²=1, yielding (x+2)²/9-(y-3)²/7=1. A common mistake is using the vertical form or incorrect center, but foci alignment determines orientation. Always verify that the center-to-focus distance equals c and that c>a for valid hyperbola geometry.

8

An ellipse has foci $F_1(-2,5)$ and $F_2(6,5)$. For every point $P$ on the ellipse, $PF_1+PF_2=18$. Which equation represents the ellipse?

$\dfrac{(x-2)^2}{81}-\dfrac{(y-5)^2}{65}=1$

$\dfrac{(x-2)^2}{81}+\dfrac{(y-5)^2}{65}=1$

$\dfrac{(x+2)^2}{81}+\dfrac{(y-5)^2}{65}=1$

$\dfrac{(x-2)^2}{65}+\dfrac{(y-5)^2}{81}=1$

Explanation

To derive this ellipse equation, we use the definition: all points where the sum of distances to foci is constant. The foci F₁(-2,5) and F₂(6,5) have the same y-coordinate, creating a horizontal ellipse centered at ((-2+6)/2,5)=(2,5). The distance between foci is 2c=8, so c=4, and PF₁+PF₂=18 gives 2a=18, so a=9. From c²=a²-b², we get 16=81-b², so b²=65. For a horizontal ellipse centered at (h,k), we use (x-h)²/a²+(y-k)²/b²=1, giving (x-2)²/81+(y-5)²/65=1. Students might place 81 under the y-term, but since foci are horizontal, a² must be under the x-term. Verify by confirming the center is equidistant from both foci and that c<a for ellipse validity.

9

An ellipse has foci $F_1(1,1)$ and $F_2(1,9)$. For every point $P$ on the ellipse, $PF_1+PF_2=16$. Which equation represents the ellipse?

$\dfrac{(x-1)^2}{64}+\dfrac{(y-5)^2}{48}=1$

$\dfrac{(y-5)^2}{64}+\dfrac{(x-1)^2}{48}=1$

$\dfrac{(y-5)^2}{64}-\dfrac{(x-1)^2}{48}=1$

$\dfrac{(y-5)^2}{16}+\dfrac{(x-1)^2}{48}=1$

Explanation

To derive this ellipse equation, we start with the geometric definition: points where the sum of distances to foci equals a constant. The foci $F_1(1,1)$ and $F_2(1,9)$ share x-coordinate 1, indicating a vertical ellipse centered at $(1, \frac{1+9}{2}) = (1,5)$. The distance between foci is $2c=8$, so $c=4$, and $PF_1+PF_2=16$ gives $2a=16$, so $a=8$. Using $c^2=a^2-b^2$, we get $16=64-b^2$, so $b^2=48$. For a vertical ellipse centered at $(h,k)$, the form is $\frac{(y-k)^2}{a^2}+\frac{(x-h)^2}{b^2}=1$, giving $\frac{(y-5)^2}{64}+\frac{(x-1)^2}{48}=1$. Students often confuse which term gets $a^2$ versus $b^2$, but for vertical foci, $a^2$ goes under the y-term. Verify by checking that the center-to-focus distance equals $c=4$ and that $a>c$ for ellipse validity.

10

A hyperbola is defined as the set of all points $P(x,y)$ such that the absolute difference of its distances to the foci $F_1(0,-5)$ and $F_2(0,5)$ is constant. Here, $|PF_2-PF_1|=6$. Which equation represents the hyperbola? (No asymptotes are given.)

$\dfrac{y^2}{9}-\dfrac{x^2}{16}=1$

$\dfrac{y^2}{25}+\dfrac{x^2}{16}=1$

$\dfrac{x^2}{9}-\dfrac{y^2}{16}=1$

$\dfrac{y^2}{16}-\dfrac{x^2}{9}=1$

Explanation

This problem requires deriving a hyperbola equation from its focus definition. A hyperbola is the set of all points P(x,y) where the absolute difference of distances to two foci is constant. The foci F₁(0,-5) and F₂(0,5) lie on the y-axis, making this a vertical hyperbola centered at the origin with c = 5. The condition |PF₂ - PF₁| = 6 means 2a = 6, so a = 3. For a hyperbola, c² = a² + b², so b² = c² - a² = 25 - 9 = 16, giving y²/9 - x²/16 = 1. A common mistake is using the ellipse relationship b² = a² - c² instead of the hyperbola relationship. Always verify that c > a for hyperbolas and use the correct formula.

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