S = `\frac{3}{1.4}` + `\frac{3}{4.7}` + `\frac{3}{7.10}` + .... + `\frac{3}{40.43}` + `\frac{3}{43/46}`
Ta có : `\frac{3}{1.4}` = `3/1 - 3/4`
`\frac{3}{4.7}`= `3/4 - 3/7`
`\frac{3}{7.10}` =` 3/7 - 3/10`
......................
`\frac{3}{40.43}` = `3/40 - 3/43`
`\frac{3}{43/46}` = `3/43 - 3/46`
⇒ S = `3/1 - 3/4` + `3/4 - 3/7` + ` 3/7 - 3/10` + ....... + `3/40 - 3/43` + `3/43 - 3/46`
`= 3/1 - 3/46`
`= 138/46 - 3/46`
`= 135/36`
Vì S =`135/36` mà `135/36` có tử bé hơn mẫu nên S < `1`
