Give an example of multiplication of 2 imaginary numbers. The result of multiplication has to be a real number.

For instance, if we'll multiply an imaginary number by
it's conjugate, we'll get a difference of squares and the result will be a real
value.

z = a + b*i and the conjugate is z' = a -
b*i

We'll apply the
formula:

(a-b*i)(a+b*i) = a^2 - b^2*i^2, but i^2 =
-1

(a-b*i)(a+b*i) = a^2 +
b^2

We'll put a = 4 and b =
2i

(4+2i)(4-2i) = 4^2 -
(2i)^2

(4+2i)(4-2i) = 16 -
4i^2

But i^2 = -1

(4+2i)(4-2i)
= 16-(-4)

(4+2i)(4-2i) =
16+4

(4+2i)(4-2i) =
20

The result of multiplication of
2 imaginary numbers, (4+2i)(4-2i), is the real number
20.