Factor by grouping

[Guest]

What am I doing wrong?

2x^3 + x^2 - 8x -4

Here is what I have done:

2x^3 + x^2 -8x-4 grouping into two groups
2x(x + 1) -8x (x-2)
(2x + 1) (x-2)


My book says the answer is (2x + 1) (x-2) (x+2)
What would you say I did wrong?????? I appreciate your help.

 

[dagr8est]

2x^3+x^2-8x-4
=(2x^3-8x)+(x^2-4)
=2x(x^2-4)+1(x^2-4)
=(2x+1)(x^2-4)
=(2x+1)(x+2)(x-2)

 

[Guest]

2x^3 + x^2 -8x-4 grouping into two groups
2x(x + 1) -8x (x-2)

Expand that second line. Is it the same as the first line? No, that's where you went wrong.

 

[Guest]

Thank you

Thanks to both. Wish I me and my son where that smart!