Question

Find the product. (z + 3i)(z − 3i)

Answer 1

Answer:

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,

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