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kozerog [31]
3 years ago
5

What is the sum of the infinite geometric series 9−6+4−83+...9-6+4-83+...??

Mathematics
1 answer:
vivado [14]3 years ago
7 0
Successive terms have a common factor of -\dfrac23, which means the nth partial sum of the series can be written as

S_n=9-6+4-\dfrac83+\cdots+9\left(-\dfrac23\right)^{n-1}
S_n=9\left(1+\left(-\dfrac23\right)^1+\left(-\dfrac23\right)^2+\left(-\dfrac23\right)^3+\cdots+\left(-\dfrac23\right)^{n-1}

\implies-\dfrac23S_n=9\left(\left(-\dfrac23\right)^1+\left(-\dfrac23\right)^2+\left(-\dfrac23\right)^3+\left(-\dfrac23\right)^4+\cdots+\left(-\dfrac23\right)^n\right)

\implies S_n-\left(-\dfrac23\right)S_n=9\left(1-\left(-\dfrac23\right)^n\right)
\dfrac53S_n=9\left(1-\left(-\dfrac23\right)^n\right)
S_n=\dfrac{27}5\left(1-\left(-\dfrac23\right)^n\right)

As n\to\infty, the geometric term vanishes, leaving you with

S=\displaystyle\lim_{n\to\infty}S_n=\dfrac{27}5
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