Question

Can someone give me the answers to these? I need to compare answers. Thanks.

http://screensnapr.com/e/jBJhX8.png

Answer 1

problem is to use the cos(a-b) identity
cos(a-b) = cosa.cosb + sina.sinb
sinb = 7/8 so think of the rightangle triangle
cos b =sqrt( (8^2 -7^2)) = sqrt15
simiiliarly
sin a = sqrt(11)/6
so cos(a-b) = sqrt15/8 * -5/6 + -7/8*sqrt11/6
-5sqrt15/48 +7sqrt(11)/48 which is c

Answer 2

actually I may have messed up my signs !! Id sin angle is negative, what does that make the cos??

Answer 3

OK if you look at the constraints B is in the third quadrant so both sin and cos are negative so the answer is right I think.
cos(a-b) = sqrt15/8 * 5/6 + 7/8 * -sqrt11/6