# How do you multiply (15+ 13i ) ( 12- 9i ), assuming i is an imaginary number?

Mar 8, 2017

$297 + 21 i$

#### Explanation:

You multiply them just like real numbers, assuming that $i \cdot i = - 1$

$\left(15 + 13 i\right) \left(12 - 9 i\right)$

$= 15 \cdot 12 + 13 i \cdot 12 - 15 \cdot 9 i - 13 i \cdot 9 i$

$= 180 + 156 i - 135 i + 117$

$= 297 + 21 i$