Đề đánh sai `31/30` thành `31/31`

`2x+7/6+13/12+21/20+31/30 + 43/42 + 57/56 + 73/72 + 91/90=10`

Đặt `A=7/6+13/12+21/20+31/30 + 43/42 + 57/56 + 73/72 + 91/90`

`A=(1+1/6)+(1+1/12)+(1+1/20)+(1+1/30)+(1+1/42)+(1+1/56)+(1+1/72)+(1+1/90)`

`A = (1+1+1+1+1+1+1+1) + (1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90)`

`A=8+(1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10)`

`A=8+(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10)`

`A=8+(1/2-1/10)`

`A=8+(5/10-1/10)`

`A=8+4/10`

`A=8+2/5`

`A= 40/5 + 2/5`

`A=42/5`

Thay `A` vào, ta có :

`2x+42/5=10`

`=> 2x = 10 - 42/5`

`=> 2x = 50/5 - 42/5`

`=> 2x = 8/5`

`=> x = 8/5 : 2`

`=> x = 8/5 . 1/2`

`=> x = 8/10`

`=> x = 4/5`

Vậy `x=4/5`