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Rzqust [24]
3 years ago
8

Use the formula to evaluate the series 1/4 + 1/9 + 1/27 + 1/81.

Mathematics
1 answer:
Dmitriy789 [7]3 years ago
8 0
Is the first term supposed to be 1/4, or did you actually mean 1/3?

I'll assume the latter. Write

S=\dfrac13+\dfrac19+\dfrac1{27}+\dfrac1{81}
S=\dfrac13+\dfrac1{3^2}+\dfrac1{3^3}+\dfrac1{3^4}
\dfrac13S=\dfrac1{3^2}+\dfrac1{3^3}+\dfrac1{3^4}+\dfrac1{3^5}
\implies S-\dfrac13S=\dfrac13-\dfrac1{3^5}
\dfrac23S=\dfrac13\left(1-\dfrac1{3^4}\right)
S=\dfrac12\left(1-\dfrac1{81}\right)
S=\dfrac{40}{81}
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