mathmatics( sequence and series)
posted by achyut on .
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 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
http://www.jiskha.com/display.cgi?id=1258559554 
the last term is 512 and common ratio is 2 then its 5th term