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6 January, 05:31

What is the 21st term of an arithmetic sequence with terms a6=50 and a11=85

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  1. 6 January, 05:55
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    A1+d (n-1)

    a1+d (6-1) = 50

    a1+5d=50

    a1+d (11-1) = 85

    a1+10d=85

    a1=50-5d

    (50-5d) + 10d=85

    50-5d+10d=85

    5d+50=85

    5d=35

    d=7

    a1+7 (6-1) = 50

    a1+35=50

    a1=15

    15+7 (n-1)

    15+7 (21-1)

    15+7 (20)

    15+140=155
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