Could someone show me why I got this problem wrong?

Question

anonymous · Answer

The problem is: $$x^4 - 2x^2 - 8$$

I originally got: (x - 4)(x + 2)

Now I have: (x - 2)(x + 2). Would this be correct?

hartnn · Answer

lets, just for simplification, put x^2 =y

so, x^4 = y^2
right ?

anonymous · Answer

Yes.

hartnn · Answer

so the equation now is 
y^2-2y-8

can u factor this ?

anonymous · Answer

I realize that's why I got it wrong the first time.

anonymous · Answer

(x - 4)(x + 2)

hartnn · Answer

you mean (y-4)(y+2)
isn't it ?

anonymous · Answer

Which would then turn into (x - 2)(x + 2). Also, the original problem had x not y.

hartnn · Answer

we put x^2 as 'y' remember ?

so, wouldn't that be 
(x^2-4)(x^2+2)
now ?

anonymous · Answer

Oh yes! I had simplified that into (x - 2)(x + 2).

hartnn · Answer

x^2-4 is indeed (x - 2)(x + 2)

so finally it would be 
(x - 2)(x + 2)(x^2+2)
got it ?

anonymous · Answer

I understand now, thank you.

hartnn · Answer

welcome ^_^