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The first term of a geometric series is 3, and the sum of the first term and the second term is 15. What is the sum of the first six terms?

User Macabeus
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Hello,

u_(1)=3

u_(2)=3*r


u_(1)+u_(2)=15=3(1+r)
==> 1+r=5==>r=4

3+3r+3r²+3r^3+...+3r^6=3(1+r+r²+...+r^6)=3*(r^7-1)/(r-1)=4^7-1=16383









User Robert Ramey
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