Question

Find the product of 9 -5i and its complex conjugate

Answer 1

(9-5i)(9+5i) = 81+25 = 106

note
(a-b)(a+b) = a^2 - b^2
but
i^2 = -1
so
(a-bi)(a+bi) = a^2-b^2i^2 = a^2+b^2