Bài 2 (4.0 điểm): Tim y biết: a) y + 1,57 = 10 - 3,4 b) ($\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + ... + $\frac{1}{110}$ + $\frac{1}{132}$) x y = 5 c) y : $\frac{1}{16}$ - y : 0,25 - 12 x y : 6 = 41,5

Đáp án + Giải thích các bước giải:

`a)y+1,57=10-3,4`

`=>y+1,57=6,6`

`=>y=6,6-1,57`

`=>y=5,03`

Vậy `y=5,03`

`b)(1/20+1/30+1/42+...+1/110+1/132)xx y=5`

`=>(1/(4xx5)+1/(5xx6)+1/(6xx7)+...+1/(11xx10)+1/(11xx12))xx y=5`

`=>(1/4-1/5+1/5-1/6+1/6-1/7+...+1/11-1/10+1/11-1/12)xx y=5`

`=>(1/4-1/12)xx y=5`

`=>(3/12-1/12)xx y=5`

`=>1/6xx y=5`

`=>y=5:1/6`

`=>y=5xx6`

`=>y=30`

Vậy `y=30`

`c)y:1/16-y:0,25-12xx y:6=41,5`

`=>y xx16-y xx4-12:6xx y=41,5`

`=>y xx16-y xx4-2xx y=41,5`

`=>y xx(16-4-2)=41,5`

`=>y xx10=41,5`

`=>y =41,5:10`

`=>y=4,15`

Vậy `y=4,15`