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March 29, 2017

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the 9th term of an AP is 52 while the 12th term is 70. find the sum of it's 20th term?

  • arithmetic progression - ,

    9th term = 52 ---> a + 8d = 52
    12th term = 70 --> a + 11d = 70
    subtract them
    3d = 18
    d = 6
    then a+48 = 52
    a = 4

    20th term = a + 19d = 4 + 114 = 118

  • arithmetic progression - ,

    118

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