Sửa đề thành : S=1+1/5+1/25+.........+1/15625
5xS=1+1/5+1/25+....+1/3125
5xS-S=(1+1/5+1/25+...+1/3125)-(1/5+1/25+1/125+....+1/15625)
4xS=1-1/15625=15624/15625
1xS=15624/15625:4=15624/15625x1/4=15624/62500=3906/15625
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\(#040510\)
\(S=1+\dfrac{1}{5}+\dfrac{1}{25}+...+\dfrac{1}{15625}\)
\(5S=5+1+\dfrac{1}{5}+...+\dfrac{1}{3125}\)
\(5S-S=5-\dfrac{1}{15625}\)
\(S=\dfrac{5.15625-1}{4.15625}\)
\(S=\dfrac{78124}{4.15625}=\dfrac{19531}{15625}\)
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