Find each exact value if 0< x < pi/2 and 0< y < pi/2.
1. cos (x+y) if sin x= 5/13 and sin y= 4/5
2. sin (x-y) if cos x= 8/17 and cos y= 3/5
3. tan (x-y) if csc x= 13/5 and cot y= 4/3
