1. The problem statement, all variables and given/known data Find the partial derivative of a*cos(xy)-y*sin(xy) with respect to y. 2. Relevant equations None. 3. The attempt at a solution The answer is -ax*sin(xy)-sin(xy)-xy*cos(xy). I know that I need to treat x as constant since I need to take the partial derivative with respect to y. But I don't know how to get -sin(xy). I know how to get -ax*sin(xy) and -xy*cos(xy).