Angle x is acute, angle y is obtuse, cos x = 15/17
and sin y = 3/5. Find sin (x + y).
After doing the math, I found sin (x + y) to be 77/85.
The textbook tells me that the answer is 13/85.
I know 13/85 would be the answer if we were looking for
sin (x MINUS y), right?