1. ## trigonometry

A and B are acute angles such that cosA=3/5 and cosB=5/12. find the value of cos(a+b). you are supposed to work it out without using your calculator.i am trying to work it out using the formula cos(A+B)=cosAcosB-sinAsinB however i dont know the value for A and B

2. ## Re: trigonometry

$\cos(a+b)=\cos(a)\cos(b) - \sin(a)\sin(b)$

you are given $\cos(a)$ and $\cos(b)$

what can you say about $\sin(a)$ and $\sin(b)$ ?

hint: think of the most common trig identity