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Algebra II : Graphing Circle Functions

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Example Question #1 : Graphing Circle Functions

The graph of the equation

$x^{2} + y^{2} + 10x - 8y = 84$

is a circle with what radius?

Possible Answers:

$5\sqrt{5}$

$10\sqrt{5}$

Correct answer:

$5\sqrt{5}$

Explanation:

Rewrite the equation of the circle in standard form

$(x - h)^{2} + \left ( y - k\right )^{2} = r^{2}$

as follows:

$x^{2} + y^{2} + 10x - 8y = 84$

$x^{2} + 10x+ y^{2} - 8y = 84$

Since $\left (\frac{10}{2} \right ) ^{2} = 5 ^{2} = 25$ and $\left (\frac{-8}{2} \right ) ^{2} = (-4)^{2} = 16$ , we complete the squares by adding:

$x^{2} + 10x+ 25 + y^{2} - 8y + 16 = 84 + 25 + 16$

$(x+5) ^{2} + \left (y- 4 \right ) ^{2} = 125$

The standard form of the equation sets

$r^{2} = 125$ ,

so the radius of the circle is

$r = \sqrt{125} = \sqrt{25} \cdot \sqrt{5} = 5 \sqrt{5}$

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Example Question #2 : Graphing Circle Functions

Determine the graph of the equation

4(x-2)^2+4(y-3)^2=1

Possible Answers:

Circle, centered at (2,3) with radius r= 1/2

Ellipse, centered at (2,3)

Circle, centered at (2,3) with radius r= 1/4

Hyperbola, centered at (2,3)

Correct answer:

Circle, centered at (2,3) with radius r= 1/2

Explanation:

The equation of a circle in standard for is:

(x-h)^2+(y-k)^2=r^2

Where the center (x,y)=(h,k) and the radius of the cirlce is .

Dividing by 4 on both sides of the equation yields

$(x-2)^2+(y-3)^2 = \frac{1}{4}$

$(x-2)^2+(y-3)^2 = \bigg(\frac{1}{2}\bigg)^2$

an equation whose graph is a circle, centered at (2,3) with radius = .5

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Example Question #3 : Graphing Circle Functions

Give the radius and the center of the circle for the equation below.

$(x-3)^{2}+(y + 4)^2 = 16$

Possible Answers:

r = 4

$\text{Center} = (-3, 4)$

r = 4

$\text{Center} = (3, 4)$

r = 16

$\text{Center} = (3, 4)$

r = 16

$\text{Center} = (3, -4)$

r = 4

$\text{Center} = (3, -4)$

Correct answer:

r = 4

$\text{Center} = (3, -4)$

Explanation:

Look at the formula for the equation of a circle below.

$(x - h)^{2}+(y -k)^{2} = r^{2}$

Here (h, k) is the center and is the radius. Notice that the subtraction in the center is part of the formula. Thus, looking at our equation it is clear that the center is (3, -4) and the radius squared is . When we square root this value we get that the radius must be .

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Example Question #4 : Graphing Circle Functions

Determine the equation of a circle whose center lies at the point $\left ( 3,4\right )$ and has a radius of .

Possible Answers:

$\frac{x^2}{3}-\frac{y^2}{4}=1$

$\frac{x^2}{3}+\frac{y^2}{4}-1=0$

(x+3)^2+(y+4)^2=1

(x-3)^2+(y-4)^2=1

Correct answer:

(x-3)^2+(y-4)^2=1

Explanation:

The equation for a circle with center (h,k) and radius is :

(x-h)^2+(y-k)^2=r^2

Our circle is centered at (3,4) with radius r=1 , so the equation for this circle is :

(x-3)^2+(y-4)^2=1

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Example Question #5 : Graphing Circle Functions

$\small f(x)=(x-2)^2+(y+5)^2=36$

What is the radius of the circle?

Possible Answers:

$\small \small r=36$

$\small r=6$

$\small \small r=2$

$\small \small r=3$

$\small \small r=5$

Correct answer:

$\small r=6$

Explanation:

The parent equation of a circle is represented by $\small f(x)=(x-h)^2+(y-k)^2=r^2$ . The radius of the circle is equal to $\small r$ . The radius of the cirle is $\small \sqrt{36}$ .

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Example Question #6 : Graphing Circle Functions

What is the center of the circle expressed by the funciton $\small x^2+4x+y^2=5$ ?

Possible Answers:

$\small (0,0)$

$\small (5,0)$

$\small (2,0)$

$\small (-5,0)$

$\small (-2,0)$

Correct answer:

$\small (-2,0)$

Explanation:

The equation can be rewritten so that it looks like the parent equation for a circle $\small (x-h)^2+(y-k)^2=r^2$ . After completeing the square, the equation changes from $\small x^2+4x+y^2=5$ to $\small x^2+4x+4+y^2=9$ . From there it can be expressed as $\small (x+2)^2+y^2=9$ . Therefore the center of the circle is at $\small (-2,0)$ .

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Algebra II : Graphing Circle Functions

Study concepts, example questions & explanations for Algebra II

All Algebra II Resources

Example Questions

Example Question #1 : Graphing Circle Functions

Example Question #2 : Graphing Circle Functions

Example Question #3 : Graphing Circle Functions

Example Question #4 : Graphing Circle Functions

Example Question #5 : Graphing Circle Functions

Example Question #6 : Graphing Circle Functions

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