Page 38 - DIGITAL BOOK MUHAMMAD AZWARDI NASUTION
P. 38

rS =                  ar +      ar +          ar +         ....+      ar    n−  2   +   ar    n−  1  +    ar   n   −
                                          2
                                                       3
          n


      S =          a +      ar +      ar +          .... +       ar    n−  2   +   ar    n−  1
                                            2
          n


    rS −          S =          ar −         a
                                    n
                      n
          n

                      ) (r −
    S   n (r −      1 =       a       n         ) 1



                         n
                 a  (r −           ) 1
    S =

        n
                      r −    1


    untuk           r      1
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