Posted by **Michael** on Wednesday, March 28, 2007 at 1:58pm.

What is the exact value of cos 40 degrees cos 50 degrees minus sin 40 degrees sin 50 degrees

do you know the formula

cos(A+b) = cosAcosB - sinAsinB

now compare it with your

cos 40 degrees cos 50 degrees minus sin 40 degrees sin 50 degrees