I'm trying to solve x^4-7x^2-1440
I've got (x^2-16)(x^2+9)0
So I get x=+-4 from the (x^2-16) part, but when I try to set (x^2+9) equal to 0 then I would have to square root -9, which can't be done - does that mean it's not factorable or that I'm doing something wrong?

Question

I'm trying to solve x^4-7x^2-144>0
I've got (x^2-16)(x^2+9)>0
So I get x=+-4 from the (x^2-16) part, but when I try to set (x^2+9) equal to 0 then I would have to square root -9, which can't be done - does that mean it's not factorable or that I'm doing something wrong?

anonymous · Answer

the solution is x>4 or x < -4

anonymous · Answer

You can also factorize and get this:

(x-4) (x+4) (x^2+9)>0
or in complex numbers notation:

(x-4) (x-3 i) (x+3 i) (x+4)>0

anonymous · Answer

You have done nothing wrong. The remaining two solutions are complex. http://www.wolframalpha.com/input/?i=x^4-7x^2-144%3E0

anonymous · Answer

"I would have to square root -9, which can't be done"

it can be done in C, not in R. It's called complex numbers

anonymous · Answer

thanks so much