Question

solve x^4-7x^2-8=0

Answer 1

you forgot -i

Answer 2

my bad\[x=\left\{ -2\sqrt{2},2\sqrt{2},-i,i \right\}\]

Answer 3

because
x^4-7x^2-8=0
(x^2+1)(x^2-8)=0
solve each separately