(a/2 - b)^2

Question

The answer they want is a^2/4 -ab+b^2. 

 

I understand the a^2/4 part and the b^2 part but where i got stuck is why the have -ab. I had something completely different. Any ideas?

Answer 1

(X - Y) squared is, in general, X squared minus 2XY + Y squared.  Memorize this basic pattern. Then use it, substituting X = a/2 and Y = b.

 

(a/2) = a²/4, BTW.