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Rajan@2021
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Find the sum of first 51 terms of an A.P whose second and third terms are 14 and 18 respectively.

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It is given that the first 51 terms of an A.P whose second and third terms are 14 and 18 respectively. Find the sum.

Arithmetic Progression chapter 9 question no 9 (ML Aggarwal, Avichal Publication)

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1 Answer

  1. a2=14 and a3=18
    Common difference, d=a3a2=1814=4
    Now
    a2=a+d=14
    a+4=14
    a=10
    Now, sum of 51 terms

    Sn=2n(2a+(n1)d)

    S51=251(2(10)+(511)4)

    =251(20+200)

    =251×220

    =51×110=5610

    S51=5610

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