Đáp án:
`C=35910`.
Giải thích các bước giải:
C`=1.2.3+2.3.4+3.4.5+...+18.19.20`
`=>`4C`=1.2.3.4+2.3.4.4+3.4.5.4+...+18.19.20.4`
`=>`4C`=1.2.3.4+2.3.4.(5-1)+3.4.5.(6-2)+...+18.19.20.(21-17)`
`=>`4C`=1.2.3.4+2.3.4.5-1.2.3.4+3.4.5.6-2.3.4.5+...+18.19.20.21-17.18.19.20`
`=>`4C`=(1.2.3.4+2.3.4.5+3.4.5.6+...+18.19.20.21)-(1.2.3.4+2.3.4.5+...+17.18.19.20)`
`=>`4C`=18.19.20.21`
`=>`C`=5.18.19.21`
`=>`C`=35910`
Vậy `C=35910`.
