Đáp án:
$-$
Giải thích các bước giải:
Câu `1` :
`A=1xx2xx3+2xx3xx4+...+99xx100xx101`
`A=1xx6+2xx12+...+99xx10100`
`Axx3=1xx6xx12+2xx12xx3+...+99xx10100xx3`
`Axx3=1xx6xx3+2xx13xx(14-11)+...+99xx10100xx(10101-10098)`
`Axx3=1xx6xx3+2xx13xx14-2xx13xx11+...+99xx10100xx10101-99xx10100xx10098`
`Axx3=99xx10100xx100981`
`Axx3=1096990200`
`A=3365663400`
Câu `2` :
`B=1xx4+2xx5+3xx6+4xx7+...+200xx203`
`Bxx3=1xx4xx3+2xx5xx3+3xx6xx3+4xx7xx3+...+200xx203xx3`
`Bxx3=1xx4xx3+2xx5xx(6-3)+3xx6xx(7-4)+4xx7xx(8-5)+...+200xx203xx(204-201)`
`Bxx3=1xx4xx3+3xx5xx6-2xx5xx3+3xx6xx7-3xx6xx4+4xx7xx8-4xx7xx5+...+200xx203xx204-200xx203xx201`
`Bxx3=200xx203xx204`
`Bxx3=8282400`
`B=2760800`
Câu `3` :
`C=1xx1+2xx2+3xx3+...+144xx144`
`Cxx3=1xx1xx3+2xx2xx3+3xx3xx3+...+144xx144xx3`
`Cxx3=1xx1xx3+2xx2xx(3-0)+3xx3xx(4-1)+...+144xx144xx(145-142)`
`Cxx3=1xx1xx3+2xx2xx3+3xx3xx4-3xx3xx1+...+144xx144xx145-144xx144xx142`
`Cxx3=144xx144xx145`
`Cxx3=3006720`
`C=1002240`
