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If 21th term of an ap …

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If 21th term of an ap is 25 then find the sum of first 41 terms ?

    • Posted by Kirti Dhillon 6 years, 10 months ago

    CBSE > Class 10 > Mathematics
    • 2 answers

    Yogita Ingle 6 years, 10 months ago

    an = a+ ( n -1)d
    a21 = 25
    a + 20d = 25
    a = 25 - 20d
    Sn = n/2 [2a + l]
    S41=(41/2)[2(25-20d)+(40d)]
    =(41/2)[50-40d+40d]
    =50(41/2)
    =1025

    2Thank You

    Mishthi ? 6 years, 10 months ago

    1025..

    2Thank You

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