15. In this problem, you will use linear algebra to rediscover a trig formla: the angle-addition rule. a) Use your answer to #5(c) to express R9 in matrix form. b) Using your answer to part (a), compute the matrix product RR9. c) If you rotate the plane by angle 6, and then by angle 1b, overall you have rotated the plane by angle 0 + 1/). This means that the composition R19 0 R9 = Ramp. Use this equation and the relationship between composition and matrix multiplication to prove the angle-sum formulas for cos and sin.