Solve using quadratic methods x4 2x2 15 0Solutionx4 2x2 1

Solve using quadratic methods: x^4 - 2x^2 -15 = 0

x^4 - 2x^2 -15 =0

Lets assume x^2 =y

y^2 -2y -15 =0

Solve using factorisation:

y^2 -5y +3y -15 =0

y( y -5) + 3( y-5) =0

(y +3)(y-5) =0

y =-3 -----> x^2 =- 3

x = i*sqrt3 , -i*sqrt3

y-5 =0

y = 5 ------> x^2 =5

x = + / sqrt5

Solution: x = isqrt3 , -isqrt3 , sqrt5 , -sqrt5