Solve the polynomial equation by finding all real roots

Question

x^4+15x^2-16

Many thanks

Answer 1

let a = x^2 ... a^2 + 15 a - 16 ... (a + 16) (a - 1)

(x^2 + 16) (x^2 - 1) ... (x + 4i) (x - 4i) (x + 1) (x - 1)