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11 June, 06:56

What is the sum of the first 11 terms of the arithmetic series in which a1 = - 12 and d=5?

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  1. 11 June, 07:14
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    Hello:

    the terme general is : an = a1 + (n-1) d

    a11 = a1 + (11-1) * 5

    a11 = - 12+50

    a11 = 38

    the sum of the first 11 terms is : S11 = (11/2) (a1+a11)

    S11 = (11/2) (-12+38)

    S11 = 143
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