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    The sum of the 1st and 2nd terms of a geometric progression is 50 and the sum of the 2nd and 3rd terms is 30. Find the sum to infinity.
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    (Original post by Multifine)
    The sum of the 1st and 2nd terms of a geometric progression is 50 and the sum of the 2nd and 3rd terms is 30. Find the sum to infinity.
    What have you tried?
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    (Original post by SeanFM;[url="tel:69964212")
    69964212[/url]]What have you tried?
    Well I used the formula for the sum of succeeding terms in a geometric progression, Sn = a(1-r^n)/1-r, substituting and getting two simultaneous equations:

    a(1-r^2)/1-r = 50 ------------1

    a(1-r^3)/1-r = 30 + a --------------2

    Now I keep getting stuck every time I attempt to substitute a variable into another . . .

    Once I can find out either a or r, I can do the rest.
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    (Original post by Multifine)
    Well I used the formula for the sum of succeeding terms in a geometric progression, Sn = a(1-r^n)/1-r, substituting and getting two simultaneous equations:

    a(1-r^2)/1-r = 50 ------------1

    a(1-r^3)/1-r = 30 + a --------------2

    Now I keep getting stuck every time I attempt to substitute a variable into another . . .

    Once I can find out either a or r, I can do the rest.
    That is tricky yes, I'd suggest going back and down another route.

    You have that a+ar = 50 and ar + ar^2 = 30. The latter is equivalent to r(a+ar) = 30.

    Does this help?
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    (Original post by SeanFM;[url="tel:69964584")
    69964584[/url]]That is tricky yes, I'd suggest going back and down another route.

    You have that a+ar = 50 and ar + ar^2 = 30. The latter is equivalent to r(a+ar) = 30.

    Does this help?
    Mm yes it does, I haven’t thought of that. Thank you, but I’m quite worried that if I had such a question in my paper, and if I wouldn’t have thought this way, I wouldn’t be able to solve it.
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    Thread moved to maths. Multifine Please post in the maths forum for maths subject questions, thanks
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    (Original post by Multifine)
    Mm yes it does, I haven’t thought of that. Thank you, but I’m quite worried that if I had such a question in my paper, and if I wouldn’t have thought this way, I wouldn’t be able to solve it.
    that is why it is important to do lots of different questions before the exams.
 
 
 
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