Ta có:
`\qquad 1/{5.6}+1/{6.7}+1/{7.8}+...+1/{48.49}+1/{49.50}`
`= 1/5-1/6+1/6-1/7+1/7-1/8+...+1/{48}-1/{49}+1/{49}-1/{50}`
`=1/5+(1/6-1/6)+(1/7-1/7)+...+(1/{49}-1/{49})-1/{50}`
`=1/5-1/{50}={10}/{50}-1/{50}`
`=9/{50}<{10}/{50}`
`<1/5<1/4`
$\\$
Ta lại có:
`\qquad 1/{5.6}+1/{7.8}+...+1/{49.50}`
`<1/{5.6}+1/{6.7}+1/{7.8}+...+1/{49.50}<1/4`
Vậy `1/{5.6}+1/{7.8}+...+1/{49.50}<1/4`
