x^4 + x^2 - 20 = 0 factor
(x^2 + 5) (x^2 - 4) = 0
(x^2 + 5) ( x - 2) ( x + 2) = 0
Set each factor to 0...and solve for x
x - 2 = 0 x + 2 = 0 x^2 + 5 = 0
x = 2 x = -2 x^2 = -5 take both roots
x = ±√-5
x = ±√5 i
So....the solutions are
x = { -2, 2, √5 i , - √5 i }