2.1 Now let us investigate whether the identity of cos(a - B) = cosacosß + sinasinßis true for all values of a and B. Let P (cosa; sina)and Q (cosß; sinß) be any two points on the circle O with radius 1. If PÔA = a and QOA = B then PÔQ = a - B 2.1.1 P(cosa; sina) O B Q(cos B; sin B) Make use of the cosine rule to determine the length of PQ. A(1:0)