$\frac{1}{3^0}$ +$\frac{1}{3^2}$ + $\frac{1}{3^4}$ +…+$\frac{1}{3^86}$

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1. Giải thích các bước giải:

    Đặt `A=1/3^0+1/3^2+…+1/3^86`

   `A=1+1/3^2+…+1/3^86`

   `=>9A=9+1+…+1/3^84`

   `=>9A-A=(9+1+…+1/3^84)-(1+1/3^2+…+1/3^86)`

   `=>8A=9-1/3^86=(3^88-1)/3^86`

   `=>A=(3^88-1)/8.3^86`

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2. Đặt `A = 1/{3^0} + 1/{3^2} + 1/{3^4} + … + 1/{3^{86}}`

   `⇔ 9A = 9 + 1 + 1/{3^2} + …. + 1/{3^{84}}`

   `⇔ 9A – A = ( 9 + 1 + 1/{3^2} + …. + 1/{3^{84}})-(1/{3^0} + 1/{3^2} + 1/{3^4} + … + 1/{3^{86}})`

   `⇔ 8A = 9 – 1/{3^{86}}`

   `⇔ A = {9-1/{3^{86}}}/{8}`.

    

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