\(P=\dfrac{1}{1+2}+\dfrac{1}{1+2+3}+\dfrac{1}{1+2+3+4}+...+\dfrac{1}{1+2+3+...+50}\)
\(P=\dfrac{1}{3}+\dfrac{1}{6}+\dfrac{1}{10}+...+\dfrac{1}{1275}\)
\(\dfrac{1}{2}P=\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{20}+...+\dfrac{1}{2550}\)
\(\dfrac{1}{2}P=\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{50.51}\)
\(\dfrac{1}{2}P=\dfrac{1}{2}-\dfrac{1}{51}=\dfrac{49}{102}\)
\(\Rightarrow P=\dfrac{49}{102}:\dfrac{1}{2}=\dfrac{49}{51}\)
49/51 mkk bị sai mất 10 điểm
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