Đáp án+Giải thích các bước giải:

 Bài `12:`

`a)(7x-11)^{3}=2^{5}.5^{2}+200`

`(7x-11)^{3}=32.25+200`

`(7x-11)^{3}=800+200`

`(7x-11)^{3}=1000`

`(7x-11)^{3}=10^{3}`

`7x-11=10`

`7x=21`

`x=21:7`

`x=3`

Vậy `x=3`

`b)(x-1)/(12)+(x-1)/(20)+(x-1)/(30)+(x-1)/(42)+(x-1)/(56)+(x-1)/(72)=16/9`

`(x-1).(1/12+1/20+1/30+1/42+1/56+1/72)=16/9`

`(x-1).((1)/(3.4)+(1)/(4.5)+(1)/(5.6)+(1)/(6.7)+(1)/(7.8)+(1)/(8.9))=16/9`

`(x-1).(1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9)=16/9`

`(x-1).[1/3+(1/4-1/4)+(1/5-1/5)+(1/6-1/6)+(1/7-1/7)+(1/8-1/8)-1/9]=16/9`

`(x-1).(1/3-1/9)=16/9`

`(x-1).(3/9-1/9)=16/9`

`(x-1). 2/9=16/9`

`x-1=16/9:2/9`

`x-1=16/2`

`x-1=8`

`x=9`

Vậy `x=9`