Question

Which pair of complex numbers has a real-number product?

O (1 + 2i) (8i)

0 (1 + 2i) (2 - 5i)

O (1 + 2i) (1 – 2i)

O (1 + 2i)(4i)

Answer 1

C

Explanation:

C

Answer 2

(1 + 2i)(1 - 2i)

Explanation:

Multiplying a complex number and its conjugate results in a real number.

Thus

1 + 2i and 1 - 2i are conjugates

(1 + 2i)(1 - 2i) ← expand using FOIL

= 1 - 2I + 2I - 4I² [ note i² = - 1 ]

= 1 - 4(- 1)

= 1 + 4

= 5 ← a real number