Factorising: The difference of squares

Hey there,

I have been factorising today..and there is something that I don't quite get, maybe I am forgetting something I have already learned since today is the second day of studying after a month of holiday. My brain doesn't seem to have adapted yet I suppose. I was doing a task where it stated "Factorise fully" Which I did the first 20 with no hassle really but then the last couple I just seem to have gotten lost..and they are all meant to be factorised fully so I don't get the difference but an example would be http://i42.tinypic.com/t9789d.jpg Now that is quite simple..the answer is (3ab + 5c)(3ab - 5c)

This is one of the ones that confused me little..They look the same and I would go about factorising it the same way as the one I did above, but the answer is different and I can't understand what was done differently, because it would seem like I could do it the same way..Example: http://i42.tinypic.com/34ditjp.jpg The answer turned out to be: http://i39.tinypic.com/2qk8l7a.jpg

OK, so howcome was that one done so differently? What has changed the way we do this one?

Any help would be appreciated! :)

Re: Factorising: The difference of squares

In the second example, both terms have the common factor a, so you want to factor that out first, leaving the other factor to be a difference of squares.