Posted by achyut on Wednesday, November 25, 2009 at 11:00am.
The sum of the first four terms of a G.P.is 30 and that of the last four terms is 960. If the first and the last term of the G.P. are 2 and 512 respectively, find the the common ratio.
The sum of n terms is given by S = a(r^n - 1)/(r - 1) where a = the first term, n = the number of terms and r = the common ratio.
Therefore, the sum of the first 4 terms, as defined, is S = 2(r^4 - 1)/(r - 1) which results in r^4 - 15r = -14.
At first inspection, r = 2.
This same question came up several days ago
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