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arithmetic progression

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If sum of first 21 terms of an arithmetic progression is 13 and that of first 13 terms is 21, show that sum of first 34 terms is -34 ?

  • arithmetic progression -

    use the first two facts to find d,a1

    Sn=n/2(2a1+(n-1)d)

    you will have two unknowns, two equations.

    then, find s34

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