a,D=1/7-1/7^2+1/7^3-1/7^4+…+1/7^51>1/8

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1. Giải thích các bước giải:

   D=1/7-1/7^2+1/7^3-1/7^4+…+1/7^51

   1/7D=1/7^2-1/7^3+1/7^4-1/7^5+…+1/7^52

   8/7D= 1/7^2-1/7^3+1/7^4-1/7^5+…+1/7^52+1/7-1/7^2+1/7^3-1/7^4+…+1/7^51

   8/7D=1/7+1/7^52

   D=8.(1/7+1/7^52)/7>1/8

    

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2. D=1/7-1/7^2+1/7^3-1/7^4+…+1/7^51

   1/7D=1/7^2-1/7^3+1/7^4-1/7^5+…+1/7^52

   8/7D= 1/7^2-1/7^3+1/7^4-1/7^5+…+1/7^52+1/7-1/7^2+1/7^3-1/7^4+…+1/7^51

   8/7D=1/7+1/7^52

   D=8.(1/7+1/7^52)/7>1/8

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