X^4 + 16 equals 0

Answer 1

x^2 = 4i or -4i

x = ±√4i or ±√-4i
= ±2√i or ±2√-i

so, what's √i?

i = 1 cis 90°
√i = 1 cis 45° = 1/√2 + 1/√2 i
similarly for -i