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Given a geometric sequence whose sum of the first 10 terms is 4 and whose sum from the 11th to the 30th term is 48, find the sum from the 31st to the 60th term.
 one year ago
 one year ago
Given a geometric sequence whose sum of the first 10 terms is 4 and whose sum from the 11th to the 30th term is 48, find the sum from the 31st to the 60th term.
 one year ago
 one year ago

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cwrw238Best ResponseYou've already chosen the best response.0
Sum of 10 = a * (r^4  1)  = 4 r 1
 one year ago

kmeds16Best ResponseYou've already chosen the best response.0
@cwrw238 why is it r^4? I thought it's r^10.
 one year ago

amistre64Best ResponseYou've already chosen the best response.0
hmm, given is: \[S_n=\frac{1r^n}{1r}\] \[S_{10}=4=\frac{1r^{10}}{1r}\] \[S_{3010}=48=\frac{1r^{20}}{1r}\]
 one year ago

shubhamsrgBest ResponseYou've already chosen the best response.2
you have been given the sum upto first 10 terms =4 you have also been given the sum upto first 30 terms = 4 + 48 =52 and you have 2 eqns with 2 variables >solve for a and r now calculate sum for first 60 terms from that subtract sum of first 30 terms.. this should help..
 one year ago

hitten101Best ResponseYou've already chosen the best response.0
dw:1351514867224:dw
 one year ago

kmeds16Best ResponseYou've already chosen the best response.0
a(r^10  1) / r  1 = 4 a(r^30  1) / r  1 = 52 ? @shubhamsrg like this?
 one year ago

hitten101Best ResponseYou've already chosen the best response.0
solve for a and r.. then find the sum of 60 terms subtract sum of 30 terms from sum of 60 terms
 one year ago

shubhamsrgBest ResponseYou've already chosen the best response.2
@kmeds16 yep @hitten101 mistake in your formulla in the denominator..
 one year ago

kmeds16Best ResponseYou've already chosen the best response.0
I got, r^10 = 3. this is confusing :/ 10th root of 3?!
 one year ago

cwrw238Best ResponseYou've already chosen the best response.0
my mistake r^10 not r^4
 one year ago

shubhamsrgBest ResponseYou've already chosen the best response.2
how'd you get that? o.O
 one year ago

hitten101Best ResponseYou've already chosen the best response.0
@kmeds16 @shubhamsrg yes no exponent in the denominator.. you are right
 one year ago

kmeds16Best ResponseYou've already chosen the best response.0
second equation divided by first equation. hehehehehe
 one year ago

shubhamsrgBest ResponseYou've already chosen the best response.2
your main aim is not to find r,, your main aim is to find sum.. leave it as r^10 = 3
 one year ago

kmeds16Best ResponseYou've already chosen the best response.0
find the sum of S60 and subtract 52, right?
 one year ago

shubhamsrgBest ResponseYou've already chosen the best response.2
do this,,this might simplify.. substitue r^10 =3 whereever you can leave r1 as it is.. you can see a/(r1) = 4/(r^10 1) in calculation for sum of 60 terms ,make use of this eqn,, no need to find a.. :)
 one year ago

shubhamsrgBest ResponseYou've already chosen the best response.2
r^60 we all can find.. hmm.. hope that helped..
 one year ago

kmeds16Best ResponseYou've already chosen the best response.0
solving...hehehe ahm, thanks for the idea..
 one year ago
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