If x=inv(tan)A and y=inv(tan)B, find tan(A+B) in terms of x and y.

I cant see how to work this out and think the question should have A and x, and B and y, interchanged.

The book has the answer (x+y)/(1-xy), which I can get if I interchange the depedent and independent variables.

Do you think that it's a print error?