a) `x/20 = 7/12 + 11/30`
`x/20 = 35/60 + 22/60`
` x/20 = 19/20`
`x= 19/20 . 20`
`x= 19`
Vậy `x= 19`
b)
Ta có: `1/2.2 < 1/1.2 ; 1/3.3 < 1/2.3; ..... ; 1/9.9 < 1/8.9`
`=> 1/2.2 + 1/3.3 + 1/4.4+.........+ 1/9.9 < 1/1.2 + 1/2.3 + .... + 1/8.9`
`=> A < 1/1 -1/2 + 1/2 -1/3 +....+ 1/8- 1/9`
`=>A < 1/1 -1/9`
`=> A < 8/9 (1)`
Lại có: `1/2.2 > 1/2.3 ; 1/3.3 > 1/3.4 ; .........; 1/9.9> 1/9.10`
`=> 1/2.2 + 1/3.3 + 1/4.4+.........+ 1/9.9> 1/2.3 + 1/3.4 +...+ 1/9.10`
`=> A > 1/2 -1/3 + 1/3 -1/4 +....+ 1/9-1/10`
`=> A > 1/2 -1/10 = 2/5(2)`
Từ `(1)` và `(2)` `=> 8/9 > A > 2/5`
Vậy `8/9 > A > 2/5`
