Originally Posted by

**sparky** I recognized it now thanks to the way you laid it out. However I did not see that before because I was trying to factorize the expression by grouping the terms in pairs so that each pair of terms has a common factor.

My question is, if I did not recognize what you pointed out, how do I factorize this expression by grouping?

For example (assuming my working is correct)

$\displaystyle \text{Factorize the following }ab(x^2+y^2)-cd(x^2+y^2)$

$\displaystyle =ab(xx+yy)-cd(xx+yy)$

$\displaystyle =(ab-cd)(xx+yy)$