Precalculus Examples

$4 x^{2} + 4 y^{2} + 160 x - 120 y = 0$

Step 1

Divide both sides of the equation by $4$ .

$x^{2} + y^{2} + 40 x - 30 y = 0$

Step 2

Complete the square for $x^{2} + 40 x$ .

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Step 2.1

Use the form $a x^{2} + b x + c$ , to find the values of $a$ , $b$ , and $c$ .

$a = 1$

$b = 40$

$c = 0$

Step 2.2

Consider the vertex form of a parabola.

$a {(x + d)}^{2} + e$

Step 2.3

Find the value of $d$ using the formula $d = \frac{b}{2 a}$ .

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Step 2.3.1

Substitute the values of $a$ and $b$ into the formula $d = \frac{b}{2 a}$ .

$d = \frac{40}{2 \cdot 1}$

Step 2.3.2

Cancel the common factor of $40$ and $2$ .

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Step 2.3.2.1

Factor $2$ out of $40$ .

$d = \frac{2 \cdot 20}{2 \cdot 1}$

Step 2.3.2.2

Cancel the common factors.

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Step 2.3.2.2.1

Factor $2$ out of $2 \cdot 1$ .

$d = \frac{2 \cdot 20}{2 (1)}$

Step 2.3.2.2.2

Cancel the common factor.

$d = \frac{2 \cdot 20}{2 \cdot 1}$

Step 2.3.2.2.3

Rewrite the expression.

$d = \frac{20}{1}$

Step 2.3.2.2.4

Divide $20$ by $1$ .

$d = 20$

Step 2.4

Find the value of $e$ using the formula $e = c - \frac{b^{2}}{4 a}$ .

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Step 2.4.1

Substitute the values of $c$ , $b$ and $a$ into the formula $e = c - \frac{b^{2}}{4 a}$ .

$e = 0 - \frac{40^{2}}{4 \cdot 1}$

Step 2.4.2

Simplify the right side.

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Step 2.4.2.1

Simplify each term.

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Step 2.4.2.1.1

Raise $40$ to the power of $2$ .

$e = 0 - \frac{1600}{4 \cdot 1}$

Step 2.4.2.1.2

Multiply $4$ by $1$ .

$e = 0 - \frac{1600}{4}$

Step 2.4.2.1.3

Divide $1600$ by $4$ .

$e = 0 - 1 \cdot 400$

Step 2.4.2.1.4

Multiply $- 1$ by $400$ .

$e = 0 - 400$

Step 2.4.2.2

Subtract $400$ from $0$ .

$e = - 400$

Step 2.5

Substitute the values of $a$ , $d$ , and $e$ into the vertex form ${(x + 20)}^{2} - 400$ .

${(x + 20)}^{2} - 400$

Step 3

Substitute ${(x + 20)}^{2} - 400$ for $x^{2} + 40 x$ in the equation $x^{2} + y^{2} + 40 x - 30 y = 0$ .

${(x + 20)}^{2} - 400 + y^{2} - 30 y = 0$

Step 4

Move $- 400$ to the right side of the equation by adding $400$ to both sides.

${(x + 20)}^{2} + y^{2} - 30 y = 0 + 400$

Step 5

Complete the square for $y^{2} - 30 y$ .

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Step 5.1

Use the form $a x^{2} + b x + c$ , to find the values of $a$ , $b$ , and $c$ .

$a = 1$

$b = - 30$

$c = 0$

Step 5.2

Consider the vertex form of a parabola.

$a {(x + d)}^{2} + e$

Step 5.3

Find the value of $d$ using the formula $d = \frac{b}{2 a}$ .

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Step 5.3.1

Substitute the values of $a$ and $b$ into the formula $d = \frac{b}{2 a}$ .

$d = \frac{- 30}{2 \cdot 1}$

Step 5.3.2

Cancel the common factor of $- 30$ and $2$ .

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Step 5.3.2.1

Factor $2$ out of $- 30$ .

$d = \frac{2 \cdot - 15}{2 \cdot 1}$

Step 5.3.2.2

Cancel the common factors.

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Step 5.3.2.2.1

Factor $2$ out of $2 \cdot 1$ .

$d = \frac{2 \cdot - 15}{2 (1)}$

Step 5.3.2.2.2

Cancel the common factor.

$d = \frac{2 \cdot - 15}{2 \cdot 1}$

Step 5.3.2.2.3

Rewrite the expression.

$d = \frac{- 15}{1}$

Step 5.3.2.2.4

Divide $- 15$ by $1$ .

$d = - 15$

Step 5.4

Find the value of $e$ using the formula $e = c - \frac{b^{2}}{4 a}$ .

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Step 5.4.1

Substitute the values of $c$ , $b$ and $a$ into the formula $e = c - \frac{b^{2}}{4 a}$ .

$e = 0 - \frac{{(- 30)}^{2}}{4 \cdot 1}$

Step 5.4.2

Simplify the right side.

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Step 5.4.2.1

Simplify each term.

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Step 5.4.2.1.1

Raise $- 30$ to the power of $2$ .

$e = 0 - \frac{900}{4 \cdot 1}$

Step 5.4.2.1.2

Multiply $4$ by $1$ .

$e = 0 - \frac{900}{4}$

Step 5.4.2.1.3

Divide $900$ by $4$ .

$e = 0 - 1 \cdot 225$

Step 5.4.2.1.4

Multiply $- 1$ by $225$ .

$e = 0 - 225$

Step 5.4.2.2

Subtract $225$ from $0$ .

$e = - 225$

Step 5.5

Substitute the values of $a$ , $d$ , and $e$ into the vertex form ${(y - 15)}^{2} - 225$ .

${(y - 15)}^{2} - 225$

Step 6

Substitute ${(y - 15)}^{2} - 225$ for $y^{2} - 30 y$ in the equation $x^{2} + y^{2} + 40 x - 30 y = 0$ .

${(x + 20)}^{2} + {(y - 15)}^{2} - 225 = 0 + 400$

Step 7

Move $- 225$ to the right side of the equation by adding $225$ to both sides.

${(x + 20)}^{2} + {(y - 15)}^{2} = 0 + 400 + 225$

Step 8

Simplify $0 + 400 + 225$ .

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Step 8.1

Add $0$ and $400$ .

${(x + 20)}^{2} + {(y - 15)}^{2} = 400 + 225$

Step 8.2

Add $400$ and $225$ .

${(x + 20)}^{2} + {(y - 15)}^{2} = 625$

Step 9

This is the form of a circle. Use this form to determine the center and radius of the circle.

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

Step 10

Match the values in this circle to those of the standard form. The variable $r$ represents the radius of the circle, $h$ represents the x-offset from the origin, and $k$ represents the y-offset from origin.

$r = 25$

$h = - 20$

$k = 15$

Step 11

The center of the circle is found at $(h, k)$ .

Center: $(- 20, 15)$

Step 12

These values represent the important values for graphing and analyzing a circle.

Center: $(- 20, 15)$

Radius: $25$

Step 13