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8 A geometric sequence has all
positive terms. The sum of the
first two terms is 15 and the sum
to infinity is 27.
a
Find the value of the common
ratio.
b Hence, find the first term.

Sagot :

Medunno13

Let the terms be a, ar, ar², ...

The sum to infinity being 27 means that:

[tex]\frac{a}{1-r}=27 \implies a=27(1-r)[/tex]

The sum of the first two terms being 15 means:

[tex]a+ar=15 \\ \\ (27-27r)+r(27-27r)=15 \\ \\ 27-27r+27r-27r^2 =15 \\ \\ 27r^2 -12=0 \\ \\ 27r^2 =12 \\ \\ r^2=\frac{12}{27}[/tex]

Since all the terms are positive, r>0.

[tex]\boxed{r=\frac{2}{3}} \\ \\ \implies a=27\left(1-\frac{2}{3} \right)=\boxed{9}[/tex]

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