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# Common Core Standard HSG-GPE.A.3 Questions

(+) Derive the equations of ellipses and hyperbolas given the foci, using the fact that the sum or difference of distances from the foci is constant.

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Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Substitute the equations for $d_1, d_2$ found in the second and third questions into the equation in the previous question. After some rearranging, the result is the following equation:

$sqrt( (x+c)^2 + y^2) = pm 2a + sqrt( (x-c)^2 + y^2)$

Why is there a plus-minus sign on the $2a$ term?
1. In order to remove the absolute value sign, one has to consider the case where $d_1>d_2$ and $d_1 < d_2$.
2. To remove the square root from this term, one has to look at the positive and negative case.
3. Since it's not certain which side of the equation this term should be on, it could be either positive or negative.
4. This is a hyperbola and the curves can be on either the positive or negative x-axis.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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It can be shown that the value $b$ from the previous question relates to the vertices of the minor-axis. Specifically, the vertices are $(0,-b)$ and $(0,b)$. Looking at the positive vertex, it forms an isosceles triangle with the two foci. What is the length of the two congruent sides? How is this related to the previous question?
1. They each have a length of $2c$. Using the distance formula and looking at the difference of distances between the lengths just found and the other vertices of the major axes, one finds that $b = sqrt(a^2-c^2$.
2. They each have a length of $c$. Then, the right triangle formed between the vertices and the origin, and applying the Pythagorean theorem, results in $a^2 + b^2 = c^2$.
3. They each have a length of $a$. Looking at the right triangle formed by the origin, $F_1$, and the vertex $(0,b)$, and applying the Pythagorean theorem results in $a^2 = b^2 + c^2$.
4. They each have a length of $a/2$. Therefore, the sum of their lengths, $a$, can be used as a value equal to the sum of the lengths of $b$ and $c$.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Which of the following is the best definition of an ellipse?
1. A squished circle.
2. An oblong shaped closed curve that has two foci.
3. Given two points, the foci, the set of all points where the sum of the the distances between a given point and the foci is constant.
4. The set of all points that are bounded by the distances from two central points, foci, and the major and minor axes.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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After some more algebra, the equation becomes:
$(c^2-a^2) x^2 - y^2 = (c^2 - a^2) y^2$
What happened to the plus-minus sign that was in the original equation in the seventh question?
1. Since there are no more square root terms, it is unnecessary.
2. Each term with it was eventually squared, and a squared term must be positive it.
3. Because the terms were moved from one side to another multiple times, it is no longer necessary.
4. Any terms with it canceled out.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Let $(x,y)$ be a point on the ellipse. Which equation represents the distance, $d_1$, between $F_1$ and the point $(x,y) ?$
1. $d_1 = sqrt( (x+c)^2 + y^2)$
2. $d_1 = sqrt( (x-c) + y^2)$
3. $d_1 = sqrt( (-x + c)^2 + y^2$
4. $d_1 = sqrt( (x-c)^2 + (y-x)^2)$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Letting $b^2 = c^2 - a^2$, the equation no becomes:
$b^2 x^2 - y^2 = b^2 a^2$
$x^2 / a^2 - y^2 / b^2 = 1$
This is the equation of a hyperbola, centered at the origin with foci $(-c,0)$ and $(c,0)$. Where does the substitution $b^2 = c^2 - a^2$ come from?
1. Since $b$ usually appears in the equation for a hyperbola, it must be included. Using the Pythagorean theorem, $a^2 +b^2 = c^2$, simply rearrange the equation.
2. It has to be done, to ensure the asymptotes are related to the equation. The equations of the asymptotes are $y= pm b/a$, and knowing that $|c| > |a|$, squaring and rearranging results in $b^2 = c^2 - a^2$.
3. It's done to simply the equation. $b$ is not defined yet, and since $|c| > |a|$, $c^2 > a^2$, and so there must be a positive number, $b^2$ such that $b^2 = c^2 - a^2$.
4. Knowing that $b$ is the length of the semi-minor axis, a right triangle can be formed with the center of the hyperbola and either foci, with $b$ as the length of one leg of this triangle. Applying the Pythagorean theorem results in $b^2 + c^2 = a^2$, and simply rearrange.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Which equation represents the distance, $d_2$, between $F_2$ and the point $(x,y) ?$
1. $d_2 = sqrt( (c-x)^2 + (y-c)^2)$
2. $d_2 = sqrt( (x-c)^2 + y^2)$
3. $d_2 = sqrt( (c-x)^2)$
4. $d_2 = sqrt( (x+c)^2 + y^2)$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Let $(-a,0)$ be the left vertex of the ellipse. What is the distance from this vertex to $F_1 ?$
1. $sqrt((c-a)^2 + y^2)$
2. $sqrt(a^2 + c^2)$
3. $a$
4. $a-c$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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What is the distance between $(-a,0)$ and $F_2 ?$
1. $a+c$
2. $c-a$
3. $a-c$
4. $sqrt(a^2 - c^2)$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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After squaring both sides, expanding, combing like terms, and dividing by any common constants, what is the resulting equation?
1. $xc - a^2 = -a sqrt( (x-c)^2+y^2)$
2. $2a^2 + c^2 + x^2 + y^2 = -2a sqrt( (x-c)^2+y^2)$
3. $-2a^2 + c^2 + x^2 + y^2 = 0$
4. $a^2 = cx$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Substituting the equations for $d_1, d_2$ found in the second and third questions into the above equation, and then moving the second square root term to the right side of the equation results in the following:
$sqrt( (x+c)^2 + y^2) = 2a - sqrt( (x-c)^2 + y^2)$
The next step will be to square both sides. Why was the second square root term moved to the right side of the equation before squaring, instead of just squaring both sides immediately?
1. It gets rid of all square root terms immediately.
2. It will make the algebra easier later on, since the two square root terms are not multiplied together.
3. This ensures that there won't be any multiple answers that usually result from square roots (taking the positive and negative).
4. This makes that equation more balanced, which looks nicer.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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After some more algebra, and then moving all terms with $x$ or $y$ to the left side of the equation (and all others to the right), the resulting equation is:

 $a^2 x^2 - c^2 x^2 + a^2 y^2$ $=$ $a^4 -a^2 c^2$ $(a^2 - c^2)x^2 + a^2 y^2$ $=$ $(a^2 - c^2) a^2$ $b^2 x^2 + a^2 y^2$ $=$ $a^2 b^2$ $x^2 / a^2 + y^2 / b^2$ $=$ $1$

This is the formula for the equation of an ellipse centered at $(0,0)$. Why can the substitution $b^2 = a^2-c^2$ be made?
1. Since $b$ is undefined so far, it can be defined as any value. Then, using the Pythagorean theorem, $a^2 + b^2 = c^2$, simply rearrange to solve for $b^2$.
2. Knowing that the semi-minor axis is $b$ units long, one can substitute the square of this value for $a^2-c^2$.
3. Since $b$ is not yet defined, it can be used to simplify the equation by defining $b^2 = a^2 - c^2$. A positive value for $a^2-c^2$ exists since $|a| > |c|$.
4. Projecting $b$, the length of the semi-minor axis, onto the semi-major axis it is seen that $b=a-c$. Then, simply square both sides of the equation.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Which of the following is the best definition of a hyperbola?
1. Two opposing parabolas.
2. Given two points, called foci, the set of points such that the absolute value of the differences of the distance between the foci and a point is constant.
3. The distances between a set of points and the foci is constant.
4. Two points, foci, whose eccentricity is greater than 1.
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Let $(x,y)$ be a point on the hyperbola. What is the distance, $d_1$, between $F_1$ and the point $(x,y) ?$
1. $d_1 = sqrt( (x+c)^2 + y^2)$
2. $d_1 = sqrt( (x-c)^2 + y^2)$
3. $d_1 = sqrt( (x^2 + y^2)$
4. $d_1 = sqrt( (x-c)^2 + (y-c)^2)$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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What is the distance, $d_2$, between the $F_2$ and the point $(x,y) ?$
1. $d_2 = sqrt( (x-c)^2 + (y-c)^2)$
2. $d_2 = sqrt( x^2 + y^2)$
3. $d_2 = sqrt( (x+c)^2 + y^2)$
4. $d_2 = sqrt( (x-c)^2 + y^2)$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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What is the distance between $F_2$ and the vertex $(a,0) ?$
1. $c$
2. $a$
3. $c-a$
4. $a+c$
Grade 11 Nonlinear Equations and Functions CCSS: HSG-GPE.A.3

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What does $|d_1 - d_2|$ equal?
1. $|d_1 - d_2| = a$
2. $|d_1 - d_2| = c$
3. $|d_1 - d_2| = |a-c|$
4. $|d_1 - d_2| = 2a$
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