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

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Rowah
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Homework Statement



[tex]28s^2 + 8st - 20t^2[/tex]

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

2. The attempt at a solution

[tex]4(7s^2 + st + st - 5t^2)[/tex]
[tex]4(7s^2 + st + st - 20t^2)[/tex]
[tex]4(s(7s + t) + t(s - 20t))[/tex]

I'm lost, can someone please help me?

Thanks in advance.
 
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In your first attempt at the solution, why not combine the second and third terms inside the (), it might make things more obvious.
 
Is there some trick to this that I am missing?

[tex]4(7s^2 + 2st - 5t^2)[/tex]
 
Well, you could note that 2st=7st-5st, which makes your expression 4(7s2+7st-5st-5t2). Is this any easier to factorise?
 
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 :)