Question 756827
cos(a - b) = cosacosb + sinasinb
cos(a + b) = cosacosb - sinasinb
= cosacosb + sinasinb -(cosacosb - sinasinb)
= cosacosb + sinasinb - cosacosb + sinasinb)
=        sinasinb + sinasinb
=             2sinasinb Proved
Hope this helps
:-)