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a. u1 = 3^1-2(1) = 3^-1 = 1/3
     u2 = 3^1-2(2) = 3^-3 = 1/27
     u2 = a.r
     1/27 = 1/3.r
      r = 1/9

b. S∞=1/3 / 1-1/9
         = 1/3 / 8/9 = 9/24