Trigonometry Help - Mathematicians needed

[GamerzInc]

Ok. I'm nearing the end of problem 5 and skipped problem 4. I have:
a^2 + 2ab + b^2 - c^2/4ab

I know the top becomes (a+b)^2 - c^2, but I don't remember why or know why I know it's that.

EDIT:
Progression goes well on problem V. I think I have like one or two more steps left, although, I'm not sure what to do. Here's my work on it thus far.

EDIT2:
Removed the image as I finished solving problem 5. Trying to tackle problem IV now.

 

[GamerzInc]

Calls for a double post

. I have finished solving all the problems. Problem IV was surprisingly very easy.

 

[xoinx]

good for you! haha i got stuck after a while... gah, that's why happens when you stop touching math for a few years...