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Math Help - Finding the radius of a cylinder

  1. #1
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    Algebra

    Hello,
    can any "smart" person who is very good at math solve this question? it is (d) in the attachment....
    good luck
    thanks
    Attached Files Attached Files
    Last edited by igcsestudent; May 14th 2011 at 09:31 AM.
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  2. #2
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    S_n is defined by S_n= \frac{1}{6}n(n+1)(n+2) so S_{n-1}= \frac{1}{6}(n-1)((n-1)+ 1)((n-1)+ 2)= \frac{1}{6}(n-1)(n)(n+1)

    You could multiply both of those out and subtract but it is simpler to notice that each has factors of 1/6, n, and n+1 andfactor those out:
    S_{n}- S_k{n-1}= \frac{1}{6}(n)(n+1)(n+2- (n-1))= \frac{1}{6}n(n+1)(3)
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  3. #3
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    sequence oroblem

    Quote Originally Posted by igcsestudent View Post
    Hello,
    can any "smart" person who is very good at math solve this question? it is (d) in the attachment....
    good luck
    thanks
    sequence 1,4,10,20,35 Find nextterm
    Differences 3,6,10, 15, next difference is 21 6 comes after 3,4,5
    35+21 is 56

    bjh
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  4. #4
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    cool!
    thanks really appreciate your help!
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