Bài `1:`
`a, C1: 125xx7+125xx3`
`=875+375`
`=1250`
----
`C2: 125xx7+125xx3`
`=125xx(7+3)`
`=125xx10`
`=1250`
-----------------
`b, C1: 234xx15+234xx10`
`=3510+2340`
`=5850`
----
`C2: 234xx15+234xx10`
`=234xx(15+10)`
`=234xx25`
`=5850`
_________________________
Bài `2:`
`a, x xx6+4xx x=20`
`x xx(6+4)=20`
`x xx10=20`
`x=20:10`
`x=2`
-----
`b, x xx25-x xx20=75`
`x xx(25-20)=75`
`x xx5=75`
`x=75:5`
`x=15`
_____________________
Bài `3:`
`a,` Sửa đề: `2021xx8+2021xx2`
`=2021xx(8+2)`
`=2021xx10`
`=20210`
-----
`b) 2021xx101-2021`
`=2021xx(101-1)`
`=2021xx100`
`=202100`
