By completing the square, find the values of x that satisfy x^4 -8x^2 +15 = 0

x^4 -8x^2 +15 = 0, we rewrite the equation in square form as (x^2-4)^2 -16 +15 =0 (x^2 -4)^2 = 1 x^2 -4 = ±1 so x^2 = 4±1, (x^2 = 3 or x^2 = 5) Therefore x = {-√3, √3, -√5, √5)