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![[Post New]](/templates/default/images/icon_minipost_new.gif) 5 Apr 2008 20:50:46 IST
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let S=1+3a+5a^2+..........nth term
> aS= a+3a^2+.............n-1 term+a*nth term
(subtracting)
S(1-a)=1+(2a+2a^2+.........n-1)-a*nth term
=1+2a(1+a^2+.........n-1)-a*nth term
= 1+2a(0)-a*nth term from property of nth root of unity
S=1-(a*nth term)/1-a
find the nth term of the series and proceed........
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