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
If A is obtuse then $\displaystyle \cos A<0\Rightarrow\cos A=-\sqrt{1-\sin^2A}$.
If B is acute then $\displaystyle \sin B>0\Rightarrow\sin B=\sqrt{1-\cos^2B}$.
Then use the identities $\displaystyle \cos(A+B)=\cos A\cos B-\sin A\sin B$
and $\displaystyle \cot(A-B)=\frac{\cot A\cot B+1}{\cot B-\cot A}$