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the 9th term of an AP is 52 while the 12th term is 70.find the sum of its 20th term

  • math -

    a+8d=52
    a+11d=70
    subtract ....
    3d = 18
    d=6

    sub into the 1st equation to find a, then
    sum(20) = (2//2)[2a + 19d ]

  • math -

    the answer is 247

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