# Math Help - trigonometry

1. ## trigonometry

Given that sin A= 3/5 and cosB= 12/13 where A is obtuse and B is acute, find the exact values of cos(A+B) and cot(A-B)

I am really having trouble understanding this question, i dont even know how to start..?

Id appreciate the help.
Thanks

2. If A is obtuse then $\cos A<0\Rightarrow\cos A=-\sqrt{1-\sin^2A}$.

If B is acute then $\sin B>0\Rightarrow\sin B=\sqrt{1-\cos^2B}$.

Then use the identities $\cos(A+B)=\cos A\cos B-\sin A\sin B$

and $\cot(A-B)=\frac{\cot A\cot B+1}{\cot B-\cot A}$

3. How do we find the values of Sin B and Cos A?

4. Hello Oasis1993
Originally Posted by Oasis1993
How do we find the values of Sin B and Cos A?
Have you really looked at red_dog's reply?