Math

Two terms of an arithmetic sequence are t6=-3 and t11=-13. What is t20?

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  1. t(6) = -3
    t(11) = -13

    -3 - (-13) = 10
    10/5 = 2

    Go one term back from last term to find term 1.

    t(10) = 10(2) + -13 = 7

    t(1) = 7
    Now you can find term 0

    7 + 2 = 9
    t(0) = 9

    Common difference is: d = -2

    Equation: tn = -2n + 9

    t(1) = 7
    t(2) = 5
    t(3) = 3
    t(4) = 1
    t(5) = -1
    t(6) = -3
    t(11) = -13
    t(20) = -31

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    posted by Chelle
  2. or, just note that T20 = T11+9d = -13-18=-31

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  3. Thank you

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    posted by Sandara

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