Đáp án:
Giải thích các bước giải:
`a) 5. (5/(1. 6)+ 5/(6. 11)+...+ 5/(26. 31))`
`= 5. (1- 1/6+ 1/6- 1/11+...+ 5/26- 5/31)`
`= 5. (1- 1/31)`
`= 5. 30/31`
`= 150/31`
`b) x+ 3/10= 4/5`
`x= 4/5- 3/10`
`x= 8/10- 3/10`
`x= 5/10`
`x= 1/2`
`c) 2/9- 3. x= -4/3`
`3. x= 2/9- (-4/3)`
`3. x= 2/9- (-12/9)`
`3. x= 14/9`
`x= 14/9: 3`
`x= 14/9. 1/3`
`x= 14/27`
`(3/5- x)+ 13/20= 5/6`
`(3/5- x)= 5/6- 13/20`
`(3/5- x)= 50/60- 39/60`
`(3/5- x)= 11/60`
`x= 3/5- 11/60`
`x= 36/60- 11/60`
`x= 25/60`
`x= 5/12`
`2x- 1/2= 3/4`
`2x= 3/4+ 1/2`
`2x= 3/4+ 2/4`
`2x= 5/4`
`x= 5/4: 2`
`x= 5/4. 1/2`
`x= 5/8`
