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What is the sum to 15 terms of an arithmetic progression whose 8th term is 4?

  • arithmetic -

    Sn = n/2 {2a + (n-1)d} then
    S15 = 15/2 {2a + 14d}
    =15/2 {2(a+7d)}
    wkt, a+7d = 15
    S15 = 15(15)=225

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