Solve Using the Quadratic Formula x^2+81=0

$x^{2} + 81 = 0$

Step 1

Use the quadratic formula to find the solutions.

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

Step 2

Substitute the values $a = 1$ , $b = 0$ , and $c = 81$ into the quadratic formula and solve for $x$ .

$\frac{0 \pm \sqrt{0^{2} - 4 \cdot (1 \cdot 81)}}{2 \cdot 1}$

Step 3

Simplify.

Simplify the numerator.

Raising $0$ to any positive power yields $0$ .

$x = \frac{0 \pm \sqrt{0 - 4 \cdot 1 \cdot 81}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot 81$ .

Multiply $- 4$ by $1$ .

$x = \frac{0 \pm \sqrt{0 - 4 \cdot 81}}{2 \cdot 1}$

Multiply $- 4$ by $81$ .

$x = \frac{0 \pm \sqrt{0 - 324}}{2 \cdot 1}$

Subtract $324$ from $0$ .

$x = \frac{0 \pm \sqrt{- 324}}{2 \cdot 1}$

Rewrite $- 324$ as $- 1 (324)$ .

$x = \frac{0 \pm \sqrt{- 1 \cdot 324}}{2 \cdot 1}$

Rewrite $\sqrt{- 1 (324)}$ as $\sqrt{- 1} \cdot \sqrt{324}$ .

$x = \frac{0 \pm \sqrt{- 1} \cdot \sqrt{324}}{2 \cdot 1}$

Rewrite $\sqrt{- 1}$ as $i$ .

$x = \frac{0 \pm i \cdot \sqrt{324}}{2 \cdot 1}$

Rewrite $324$ as $18^{2}$ .

$x = \frac{0 \pm i \cdot \sqrt{18^{2}}}{2 \cdot 1}$

Pull terms out from under the radical, assuming positive real numbers.

$x = \frac{0 \pm i \cdot 18}{2 \cdot 1}$

Move $18$ to the left of $i$ .

$x = \frac{0 \pm 18 i}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{0 \pm 18 i}{2}$

Simplify $\frac{0 \pm 18 i}{2}$ .

$x = \pm 9 i$