Question

find the value of x^3+2x^2-3x+21;x=1+2i​

Answer 1

x^3 + 2x^2 - 3x + 21

x^2( x + 2) - (3x - 21)

given

x = 2i + 1

x^2 = -4 + 1 + 4i

x^2 = 4i - 3

(4i - 3)(2i +3) - (6i - 21)

- 8 + 12i - 6i - 9 - 6i + 21

21 - 17

4