1. The problem statement, all variables and given/known data Find y such that cos y=cos(90+y) and −90degrees <= y <= 0degrees? 2. Relevant equations cos(a+b)=sin(a)cos(b)+cos(a)sin(y) 3. The attempt at a solution This is a question that came up in my linear algebra class. We just learned about dot product, but this doesn't seem like it's related. It almost seems like a straight trig question. Anyways, is this a trick question? Wouldn't the answer be 0 since you can't take the cosine of a negative number?