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    hi can someone help me on this question ;-

    n sum r=1 (4r-3) >2000?? i got 23 but my book is getting 36 please help thanks
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    please anyone
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    somebody?
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    What have you done so far?
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    (Original post by ghostwalker)
    I get n=32
    how cause i got a quadratic formula
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    (Original post by msmith2512)
    What have you done so far?
    can u show me bro
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    I agree with ghostwalker and 32.

    If you want some help then show me what you have done so far and I might be able to help you understand. Just giving you the answer won't help you in future.
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    (Original post by msmith2512)
    I agree with ghostwalker and 32.

    If you want some help then show me what you have done so far and I might be able to help you understand. Just giving you the answer won't help you in future.
    ok i got for the first 3 terms 1,5,9
    then i used the sum formula
    sn=n/2(2+4n-4)=2000
    sn=n(8n-4)=4000
    8n squared -4n-4000
    quadratic formula so i got 23
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    (Original post by otrivine)
    ok i got for the first 3 terms 1,5,9
    then i used the sum formula
    sn=n/2(2+4n-4)=2000
    OK up to here.

    sn=n(8n-4)=4000
    At this point you've multiplied by 4 on the LHS (you got rid of the 2 and multiplied the bracket by 2), and by 2 on the RHS.
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    (Original post by ghostwalker)
    OK up to here.



    At this point you've multiplied by 4 on the LHS (you got rid of the 2 and multiplied the bracket by 2), and by 2 on the RHS.
    Yes is that correct?
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    (Original post by otrivine)
    Yes is that correct?
    No.

    What ever you do to one side of an equation, you must do to the other to maintain equality.

    So, multiplying one side by 4, and the other by 2 isn't a valid option.

    I'd suggest just simplifying the LHS and dividing what you have in the brackets by the 2.
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    (Original post by ghostwalker)
    No.

    What ever you do to one side of an equation, you must do to the other to maintain equality.

    So, multiplying one side by 4, and the other by 2 isn't a valid option.

    I'd suggest just simplifying the LHS and dividing what you have in the brackets by the 2.
    So u mean multiply both sides by 2 but I did right?
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    (Original post by otrivine)
    So u mean multiply both sides by 2 but I did right?
    No, you multiplied one side by 4.
    Looking at one of two of your other threads, it looks like you're struggling with basic algebraic maniplulation; and that's an issue you really need to address.

    I'll post the working for this one using two different methods.

    Starting from S_n=\frac{n}{2}(2+4n-4)=2000

    Let's simplify that to \frac{n}{2}(4n-2)=2000--------------(X)

    A)

    If we multiply through by 2 on each side, we get:

    n(4n-2)=4000

    Then expanding brackets: 4n^2-2n=4000

    And we can divide through by 2 and rearrange to get 2n^2-n-2000=0

    B) Alternatively, from (X), we could have

    divided the 2 into the brackets to get

    n(2n-1)=2000

    Expanding the brackets now, and rearranging we have, as before.

    2n^2-n-2000=0
 
 
 

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