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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?

Respuesta :

caylus
Hello,
[tex] u_{1}=3 [/tex]
[tex] u_{2}=3*r [/tex]

[tex] u_{1}+u_{2}=15=3(1+r) [/tex]
==> 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









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