`3/4x6` + `3/6x8`+ ...... + `3/118x120`
=> Đáp án + Lời giải :
Gọi A = `3/4x6 + `3/6x8` + ...... + `3/118x120`
=> `1/3`A = `1/4x6` + `1/6x8` + ...... + `1/118x120`
=> `2/3` A = `2/4x6` + `2/6x8` + ...... + `2/118x120`
=> `2/3` A = `6-4/4x6` + `8-6/6x8` + ... + `120 - 118 / 118x120`
=> `2/3` A = `6/4x6` - `4/4x6` + `8/6x8` - `6/6x8` + ... + `120/118x120` - `118/118x120`
=> `2/3` A = `1/4` - `1/6` + `1/6` - `1/8` + ... + `1/118` - `1/120`
=> `2/3` A = `1/4` - `1/120`
=> A = ( `1/4` - `1/120 ) x 3 : 2
Vậy A = $\frac{(`1/4` - `1/120`) x 3}{2}$
