Question

Find the product.
(z + 3i)(z − 3i)

Answer 1

First, we use distributive property:

`(z+3i)(z-3i)=z^2-3zi+3zi-9i^2`

Now, remember that
`i^2=-1`, then

`z^2-3zi+3zi-9i^2=z^2-9(-1)=z^2+9`

Then,

`(z+3i)(z-3i)=z^2+9`