Question 836855
0 =x^4+3x^2-40
0 = (x^2 + 8)(x^2 - 5)
So x^2 + 8 = 0 or x^2 - 5 = 0
Take x^2 + 8 = 0 
x^2 = -8
x = {{{sqrt(-8)}}}
x = {{{-2sqrt(2)i}}} or {{{2sqrt(2)i}}}
For x^2 - 5 = 0
x^2 = 5
x = {{{-sqrt(5)}}} or {{{sqrt(5)}}}