#1**+3 **

a) x^4 - x^2 - 20 = 0

We can actually factor this one in a straightforward manner thusly

(x^2 - 5) ( x^2 + 4) = 0

This means that

x^2 - 5 = 0 add 5 to both sides

x^2 = 5 take both roots

x = ±√5

Also

x^2 + 4 = 0

x^2 = -4 take both roots

x = ±√[ -4]

x = ± 2i

And those are the four solutions.....

CPhill Feb 19, 2019