Differentiation anomoly--in need of setting straight! I was studying for the AP calculus exam when I came accross this question, which seems to have multiple answers. I cannot seem to find what is wrong with it (nor can my calc teacher). Find dy/dx if x + y = x * y I went about this problem the "easy way out". Instead of implicitly differentiating, I figured I could do some algebra to simplify it to y = x / (x-1). I saw the answers, and only one of them did not have a "y" in the answer, so I picked that one. It was wrong. When solved implicitly, a different answer is found: (y-1)/(1-x). This is STILL different than what our answer key has, which is (1-y)/(x-1). Can someone please help resolve this? Thanks!