How can I solve t^4 - 11t^2 + 18 = 0

Answer 1

let z = t^2

z^2 - 11 z + 18 = 0
(z-9)(z-2) = 0
z = 9 or 2
so
t^2 = 9
t = 3 or -3
t^2 = 2
t = sqrt 2 or -sqrt 2