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t11=19 and t17=61.then what will be S12?

arithmetic series question

  • math -

    a+10d = 19
    a+16d = 61
    subtract them
    6d = 42
    d = 7
    a + 70 = 19
    a = -51

    sum(12) = (12/2)(-102 + 11(7) ) = -150

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