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    How to prove the sum of a geometric sequence

    i got this so far
    S_n = a+ar+ar^2 .........ar^{n-1}

    rS_n=ar+ar^2 + ar^3 ......ar^{n-1} +ar^n

    what to do next?
    Spoiler:
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    Of course i've proved it i just want someone to show me why they do each step instead of me brainlessly proving it without understand doing it xD
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    (Original post by thefatone)
    How to prove the sum of a geometric sequence

    i got this so far
    S_n = a+ar+ar^2 .........ar^{n-1}

    rS_n=ar+ar^2 + ar^3 ......ar^{n-1} +ar^n

    what to do next?
    Spoiler:
    Show
    Of course i've proved it i just want someone to show me why they do each step instead of me brainlessly proving it without understand doing it xD
    Subtract the two so that most of the terms cancel out.
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    (Original post by Zacken)
    Subtract the two so that most of the terms cancel out.
    ok
    lets call rSn 2 and Sn 1
    1-2
    S_n -rS_n= a+ar^n???
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    (Original post by thefatone)
    ok
    lets call rSn 2 and Sn 1
    1-2
    S_n -rS_n= a+ar^n???
    No. Try again, what does subtraction mean? Do you understand the difference between subtraction and addition? How did you get +ar^n?
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    (Original post by Zacken)
    No. Try again, what does subtraction mean? Do you understand the difference between subtraction and addition? How did you get +ar^n?
    To me :facepalm:

    right i see so it must be -ar^n thanks for clearing that up
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    (Original post by thefatone)
    To me :facepalm:

    right i see so it must be -ar^n thanks for clearing that up
    Yep.
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    (Original post by Zacken)
    Yep.
    phew that was close almost asked for help on the integration question(8) then realised i made a mistake on the first bit
    where i said

    the gradient of the normal is \dfrac{1}{2}


    when i said earlier on\dfrac{\mathrm d y}{\mathrm d x} =-1

    at least i didn't get it wrong and the last question was a show ^-^ what a lifesaver "show" is....
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    (Original post by thefatone)
    phew that was close almost asked for help on the integration question(8) then realised i made a mistake on the first bit
    where i said

    the gradient of the normal is \dfrac{1}{2}


    when i said earlier on\dfrac{\mathrm d y}{\mathrm d x} =-1

    at least i didn't get it wrong and the last question was a show ^-^ what a lifesaver "show" is....
    Good going.
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    (Original post by Zacken)
    Good going.
    phew hopefully 100% in this paper ....

    hope
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    Good job.
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    (Original post by Zacken)
    Good job.
    https://b940eb267c31463d4d23ccfd3105...%20Edexcel.pdf
    question 3

    where do they get the y value of 3 from????
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    (Original post by thefatone)
    https://b940eb267c31463d4d23ccfd3105...%20Edexcel.pdf
    question 3

    where do they get the y value of 3 from????
    Uhm... you have the equation y = x + 4/x^2 that gives you the y coordinate for every x coordinate you plug in...
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    (Original post by Zacken)
    Uhm... you have the equation y = x + 4/x^2 that gives you the y coordinate for every x coordinate you plug in...
    yes so i got 2 as an x-cood oh poop....
    lost another mark there *cries
 
 
 
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