tính 1) 2/1.4 + 2/2.6 + 2/3.8 + … + 2/49.100 2) 2/1.6 + 2/2.9 + 2/3.12 + 2/4.15 + 2/5.18 + 2/6.21

Đáp án:

`1`

`2/1.4 + 2/2.6 + 2/3.8 + … + 2/49.100`

`=1/1.2+1/2.3+1/3.4+…+1/49.50`

`=1-1/2+1/2-1/3+1/3-1/4+…+1/49-1/50`

`=1-1/50`

`=49/50`

`2`

`A=2/1.6 + 2/2.9 + 2/3.12 + 2/4.15 + 2/5.18 + 2/6.21`

`=>A/2=1/1.6+1/2.9+1/3.12+1/4.15+1/5.18+1/6.21`

`=>(3A)/2=3/1.6+3/2.9+3/3.12+3/4.15+3/5.18+3/6.21`

`=>(3A)/2=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7`

`=>(3A)/2=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7`

`=>(3A)/2=1-1/7`

`=>(3A)/2=6/7`

`=>3A=12/7`

`=>A=4/7`

Vậy biểu thức có giá trị là `4/7`.