Simplify -4i*5i

$- 4 i \cdot 5 i$

Multiply $- 4 i (5 i)$ .

Multiply $5$ by $- 4$ .

$- 20 i i$

Raise $i$ to the power of $1$ .

$- 20 (i^{1} i)$

Raise $i$ to the power of $1$ .

$- 20 (i^{1} i^{1})$

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$- 20 i^{1 + 1}$

Add $1$ and $1$ .

$- 20 i^{2}$

Rewrite $i^{2}$ as $- 1$ .

$- 20 \cdot - 1$

Multiply $- 20$ by $- 1$ .

$20$