Posted by kingsley on Wednesday, October 5, 2011 at 6:15am.
the 3rd term of a g.p is 63 while the 5th term is 567,what is the sum of the first 7 terms

maths  Reiny, Wednesday, October 5, 2011 at 8:39am
t(5) = ar^4 = 567
t(3) = ar^2 = 63
divide first equation by the second
ar^4/(ar^2) = 567/63
r^2 = 9
r = ± 3
if r = ±3,
ar^2= 63
a = 7
Since you only want 7 terms, it would probably be faster to just find them and add them up
that is,
7 + 21 + 63 + 189 + 567 + 1701 + 5103 = 7651
or 7  21 + 63  189 + 567  1701  5103 = ......
or by formula
if a=7, r= 3
sum(7) = 7(3^7  1)/(31) = 7651
or if x=3
sum(7) = 7((3)^7  1)/(31) = .....

maths  Anonymous, Sunday, January 25, 2015 at 10:56am
The third term of a g p is 63 while the 5th term is 567 what is the sum of the first seven terms
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