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Thread: Basic Problem

  1. October 26th 2007, 04:23 AM #1
    Pom
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    Exclamation Basic Problem

    the series 5, 9, 14, 20
    what is the equation for the nth term????
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  2. October 26th 2007, 09:21 AM #2
    red_dog
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    a_1=5
    a_2=9=5+4=a_1+1+3
    a_3=14=9+5=a_2+2+3
    a_4=20=14+6=a_3+3+3
    .................................................
    So, a_n=a_{n-1}+(n-1)+3
    Adding the equalities we have
    a_n=5+[1+2+3+\ldots+(n-1)]+3(n-1)=5+\frac{n(n-1)}{2}+3(n-1)
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