1/2-1/3+1/4-1/5+...+1/48 -1/49 so sánh với 2/5
`=` ( $\dfrac{1}{2}$ `-` $\dfrac{1}{3}$ `+` $\dfrac{1}{4}$ `-` $\dfrac{1}{5}$ `+` $\dfrac{1}{6}$ ) `-` ( $\dfrac{1}{7}$ `-` $\dfrac{1}{8}$ ) `-` ....... `-` ( $\dfrac{1}{47}$ `-` $\dfrac{1}{50}$ )
`=` $\dfrac{23}{60}$ `......` `-` $\dfrac{1}{2256}$ `-` $\dfrac{1}{50}$ `<` $\dfrac{24}{60}$ `=` $\dfrac{2}{5}$ .
`=` A `<` $\dfrac{2}{5}$ .
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