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0,4,18,48,100.....

How can I find the general term for this ?

  • math -

    I found that the third differences were a constant of 6, so we have a cubic.

    assume the function is
    f(x) = ax^3 + bx^2 + cx + d

    for (1,0) --> a + b + c + d = 0 ..... #1
    for (2,4) --> 8a + 4b + 2c + d = 4 ....#2
    for (3,18) -> 27a + 9b + 3c + d = 18 .. #3
    for (4,48) -> 64a + 16b + 4c + d = 48 ... #4

    #2 - #1 --> 7a + 3b + c = 4 .....#5
    #3 - #2 --> 19a + 5b + c = 14 ....#6
    #4 - #3 --> 37a + 7b + c = 30 ....#7

    #6 - #5 ---> 12a + 2b = 10 ----> 6a + b = 5
    #7 - #6 ---> 18a + 2b = 16 ----> 9a + b = 8

    subtract these last two:
    3a = 3
    a = 1

    in 6a + b = 5 ---> 6+b=5 , ---- b = -1

    in #5: 7a + 3b + c = 4
    7 - 3 + c = 4 ------------- c = 0

    in #1:
    a + b + c + d = 0
    1 - 1 + 0 + d = 0
    d = 0

    so our general term is
    x^3 - x^2

    or
    term(n) = n^3 - n^
    = n^2 (n-1) , where n ∈ N

    testing:
    term(1) = 1^3 - 1^2 = 0
    term(2) = 2^3 - 2^2 = 4
    term(3) = 3^3 - 3^2 = 27-9 = 18
    term(4) = 4^3 - 4^2 = 64-16 = 48
    term(5) = 5^3 - 5^2 = 125 - 25 = 100

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