Solve Using the Quadratic Formula y^2-8y=-9

$y^{2} - 8 y = - 9$

Step 1

Add $9$ to both sides of the equation.

$y^{2} - 8 y + 9 = 0$

Step 2

Use the quadratic formula to find the solutions.

$\frac{- b \pm \sqrt{b^{2} - 4 (a c)}}{2 a}$

Step 3

Substitute the values $a = 1$ , $b = - 8$ , and $c = 9$ into the quadratic formula and solve for $y$ .

$\frac{8 \pm \sqrt{{(- 8)}^{2} - 4 \cdot (1 \cdot 9)}}{2 \cdot 1}$

Step 4

Simplify.

Simplify the numerator.

Raise $- 8$ to the power of $2$ .

$y = \frac{8 \pm \sqrt{64 - 4 \cdot 1 \cdot 9}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot 9$ .

Multiply $- 4$ by $1$ .

$y = \frac{8 \pm \sqrt{64 - 4 \cdot 9}}{2 \cdot 1}$

Multiply $- 4$ by $9$ .

$y = \frac{8 \pm \sqrt{64 - 36}}{2 \cdot 1}$

Subtract $36$ from $64$ .

$y = \frac{8 \pm \sqrt{28}}{2 \cdot 1}$

Rewrite $28$ as $2^{2} \cdot 7$ .

Factor $4$ out of $28$ .

$y = \frac{8 \pm \sqrt{4 (7)}}{2 \cdot 1}$

Rewrite $4$ as $2^{2}$ .

$y = \frac{8 \pm \sqrt{2^{2} \cdot 7}}{2 \cdot 1}$

Pull terms out from under the radical.

$y = \frac{8 \pm 2 \sqrt{7}}{2 \cdot 1}$

Multiply $2$ by $1$ .

$y = \frac{8 \pm 2 \sqrt{7}}{2}$

Simplify $\frac{8 \pm 2 \sqrt{7}}{2}$ .

$y = 4 \pm \sqrt{7}$

Step 5

The result can be shown in multiple forms.

Exact Form:

$y = 4 \pm \sqrt{7}$

Decimal Form:

$y = 6.64575131 \dots, 1.35424868 \dots$