Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have an equation I have been given to solve, I know how to start but I do not know what to do after I use the Trig Identities. Any help?

Here is what I was given $$ \frac{\cos(A + B) + \cos(A - B)}{\sin A \sin B} $$ I got to this step $$ \frac{\cos A\cos B-\sin A\sin B + \cos A\cos B+\sin A\sin B}{\sin A\sin B} $$ What do I do next to simplify?

share|cite|improve this question
$\sin a\sin b-\sin a\sin b=0$ – NotNotLogical Apr 16 '14 at 18:22
up vote 1 down vote accepted

\begin{align} \cos(A+B) + \cos(A-B) & = \cos(A) \cos(B) - \sin(A) \sin(B) + \cos(A) \cos(B) + \sin(A) \sin(B)\\ & = 2 \cos(A) \cos(B) \end{align} Hence, $$\dfrac{\cos(A+B) + \cos(A-B)}{\sin(A) \sin(B)} = 2 \cot(A) \cot(B)$$

share|cite|improve this answer
Thank you so much – need help Apr 16 '14 at 18:25
I'll accept your answer in a few minutes when it allows me – need help Apr 16 '14 at 18:26

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.