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## Homework Statement

Hi, I have an exam tomorrow and I'm trying to do the following question which I've almost solved:

Find the perfect differential of the following:

[(x² + 2xy) / (x + y)² ]dx - [x² / (x + y)² ]dy

I differentiated the first term with respect to 'y' and then the second term with respect to x, equated them but they weren't the same, what could I be doing wrong?

The problem is I can't seem to get both terms to equal each other unless i cancel the denominator..but why can't i get them to equal each other if i differentiate with the denominators too? Should be the same...?