Ta có
A=1/5+1/5^2+1/5^3+...+1/5^101
=>5A=5(1/5+1/5^2+1/5^3+...+1/5^101)
= 1+1/5+1/5^2+...+1/5^100
=>5A-A= (1+1/5+1/5^2+...+1/5^100) - (1/5+1/5^2+1/5^3+...+1/5^101)
4A=1-1/5^101
=> A=(1-1/5^101)/4
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