Solve the equation x^4-13x^2+36=0 by recognizing it as a disguised quadratic equation?

Answer 1

let's make a substitution
u=x^2

the equation is now:
u^2-13u+36=0
(u-9)(u-4)=0
u=9 or u=4
x^2=9 or x^2=4
x=-3,3,-2,2

Answer 2

x^4 - 13x^2 + 36
(x^2 - 9)(x^2 - 4)
(x - 3)(x + 3)(x - 2)(x + 2)
x = 3, -3, 2, or -2