Please review the sum and difference formulas for the trig functions.
In this case you have the difference between two angles u and v and want to find the cosine of this difference.
Here's the appropriate formula:
cos (a-b) = cos a cos b + sin a sin b
Given that sin u = -9/41, equal to the opp side over the hyp., we must find the length of the adj. side and then u itself.
opp side = - 9
hyp = 4
adj side = sqrt( 18+4) = sqrt(22)
Find the side opp angle v in a similar manner.
Then cos (u-v) = cos u cos v + sin u sin v becomes
= (cos u)(15/17) + (-9/41)(sin v) (where you must find cos u and sin v)
See what you can do, then return and comment here if you need further help. Good luck.
