Hi I've got some maths homework and stuck on the last 3 questions.

1. Given that sinA=3/5 and cosB=12/13, where A is obtuse and B is acute, find exact values of

a) cos(A+B)

b)cot (A-B)

2. Prove cot(A+B)is identical tocotAcotB-1

cotA+cotB

3. prove that tanA+tanB is identical tosin(A+B)

cosAcosB

I have tried all these and just get stuck .

any help would be really appreciative

thankyou