Question

What is the product of these numbers?

(1 + 2i)(2 + i)


1 − 5i


1 + 5i


0 − 5i


0 + 5i

Answer 1

0 + 5i

Explanation:

Multiply these the way you would any binomials, then replace i² with -1.

(1 +2i)(2 +i) = 1·2 +1·i +(2i)·2 +(2i)·i

= 2 + i + 4i + 2i² . . . . . the partial products

= 2 +5i -2 . . . . . . . . . . after replacement of i² = -1

= 0 +5i