Evaluate 4 square root of -81

$4 \sqrt{- 81}$

Step 1

Rewrite $- 81$ as $- 1 (81)$ .

$4 \sqrt{- 1 (81)}$

Step 2

Rewrite $\sqrt{- 1 (81)}$ as $\sqrt{- 1} \cdot \sqrt{81}$ .

$4 (\sqrt{- 1} \cdot \sqrt{81})$

Step 3

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

$4 (i \cdot \sqrt{81})$

Step 4

Rewrite $81$ as $9^{2}$ .

$4 (i \cdot \sqrt{9^{2}})$

Step 5

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

$4 (i \cdot 9)$

Step 6

Move $9$ to the left of $i$ .

$4 (9 \cdot i)$

Step 7

Multiply $9$ by $4$ .

$36 i$