# If $\cos (A-B)=3/5$ and $\tan A \tan B=2$. then which one of the following is true?

$\begin {array} {1 1} (a)\;\sin (A+B)=\frac{1}{5} & \quad (b)\;\sin (A+B)=-\frac{1}{5} \\ (c)\;\cos (A-B)=\frac{1}{5} & \quad (d)\;\cos (A+B)=-\frac{1}{5} \end {array}$

$(d)\;\cos (A+B)=-\frac{1}{5}$
answered Nov 7, 2013 by