# Simple Factoring Problem

1. Feb 20, 2007

### Rowah

1. The problem statement, all variables and given/known data

$$28s^2 + 8st - 20t^2$$

The answer in the back of the book is 4(7s - 5t)(s + t)

2. The attempt at a solution

$$4(7s^2 + st + st - 5t^2)$$
$$4(7s^2 + st + st - 20t^2)$$
$$4(s(7s + t) + t(s - 20t))$$

Last edited: Feb 20, 2007
2. Feb 20, 2007

### robb_

In your first attempt at the solution, why not combine the second and third terms inside the (), it might make things more obvious.

3. Feb 20, 2007

### Rowah

Is there some trick to this that I am missing?

$$4(7s^2 + 2st - 5t^2)$$

4. Feb 20, 2007

### cristo

Staff Emeritus
Well, you could note that 2st=7st-5st, which makes your expression 4(7s2+7st-5st-5t2). Is this any easier to factorise?

5. Feb 20, 2007

### Rowah

OHH OHH OHHHH OHHHHHH!!!

Oh my god, how could I be so blind? That t^2 at the end threw me off. I feel like such an idiot. Thanks a billion cristo and robb, and I apologize for wasting your time :)