`#Mon`
`B=1.4+4.7+7.10+...+97.100`
`=9B=1.4.9+4.7.9+7.10.9+...+97.100.9`
`=>9B=1.4(2+7)+4.7(10-1)+7.10(13-4)+...+97.100(103-94)`
`=>9B=1.2.4+1.4.7-1.4.7+4.7.10-4.7.10+7.10.13+...-94.97.100+97.100.103`
`=>9B=1.2.4+97.100.103`
`=>9B=8+999100=999108`
`=>B=111012`
`S=1^2+2^2+3^2+...+n^2`
`=1(2-1)+2(3-1)+3(4-1)+...+n[(n+1)-1]`
`=1.2+2.3+3.4+...+n(n+1)-(1+2+3+...+n)`
`Đặt A=1.2+2.3+...+n(n+1)`
`=>3A=1.2.3+2.3(4-1)+...+n(n+1)[(n+2)-(n-1)]`
`=1.2.3+2.3.4+...+n(n+1)(n+2)-(1.2.3+2.3.4+(n-1)n(n+1)]`
`=n(n+1)(n+2)`
`=>A=\frac{1}{3}n(n+1)(n+2)`
`=>S=\frac{1}{3}n(n+1)(n+2)-\frac{1}{2}n(n+1)`
`=\frac{2n(n+1)(n+2)-3n(n+1)}{6}`
`=\frac{n(n+1)(2n+1)}{6}`
