1. The problem statement, all variables and given/known data The two vectors a and b lie in the xy plane and make angles (alpha and beta) with the x axis. a. by evaluating a dot b in two ways prove the well known trig identity cos(alpha-beta)=cosalphacosbeta +sinalphasinbeta 2. Relevant equations adotb=abcostheta=axbx+ayby 3. The attempt at a solution Could not really come to the proper set-up.