1-18. If a, r are two real numbers, we know that York _ a(1 - yotl Use this to determine the 1-r following sums: (a) (e) K= 1 IM - IMI n-1 (b) ark (d) K=D 1-19. Evaluate each of the following sums: 10 (a) (2* + 3*+2) (b) [(-3)34 (c) _ [4*+3 + 4] *=0 * = -2 1-20. (a) Suppose r is a real number with |r|