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Posted on Nov 04, 2010

Sum to n terms of the series 1+5+12+22..........

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  • Posted on Jan 06, 2011
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You did not provide a general (n) term. However, I figured it to be something involving n^2. Hence the sum to n terms can be of the form: xn^3 + yn^2 + zn. Using the partial sums: 1, 6 (1+5), and 18 (1+5+12) we can build a linear equations system:
1= x + y +z (n=1)
6 = 8x + 4y +2z (n=2, n^2=4, n^3 = 8)
18 = 27x + 9y + 3z (n=3, n^=9, n^=27)
Solving for x,y,z we get x = 1/2, y= 1/2, z =0
Hence 1+5+12+22+..+ f(n^2)= 1/2(n^3+n^2)

  • Anonymous Jan 07, 2011

    The nth term function, f(n) is (3/2n^2 - 1/2n).
    1 5 12 22 ..
    1st difference: 4 7 10 1st 1st diff = 4
    2nd difference: 3 3 1st 2nd diff = 3
    f(n) = 1st term (n-1)/1*(1st 1st difference) (n-1(n-2)/(1*2)*(1st 2nd difference)

    f(n) = 1 (n-1)/1*4 (n-1(n-2)/(1*2)*3 = (3/2)*n^2 - (1/2)*n

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oneplusgh_15.jpg
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oneplusgh.gif

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oneplusgh_26.jpg


The summation being preformed on the Ti 89. The actual summation was preformed earlier. I just wanted to show the symbolic value of (n) in both calculations. All I need to do is drop the summation sign to the actual calculation and, fill in the term value (k), for each binomial coefficient.



oneplusgh_18.jpg

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oneplusgh_19.jpg

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oneplusgh_20.jpg


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oneplusgh_27.jpg



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oneplusgh_28.jpg



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oneplusgh_21.jpg


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oneplusgh_22.jpg

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oneplusgh_23.jpg

Putting the coefficients together was equal or, the same as for when I used the expand command on the Ti 89.

binomial coefficient (n over k) for (x+y)^6
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