樓主我打得有點(diǎn)雜亂,你可以抄寫在紙上再看
9999×10001-9996×10002
=(9996+3)×10001-9996×(10001+1)
=9996×10001+3×10001-996×10001-9996
=30003-9996
=20007
先補(bǔ)充個公式：1²+2²+3²+……+n²=n×(n+1)×(2n+1)÷6
1+(1+2)+(1+2+3)+(1+2+3+4)+……+(1+2+3+4+……+99)
=(1+1)×1÷2+(1+2)×2÷2+(1+3)×3÷2+(1+4)×4÷2+……+(1+99)×99÷2
=[(1+1²)+(2+2²)+(3+3²)+(4+4²)+……+(99+99²)]÷2
=[(1+2+3+4+……+99)+(1²+2²+3²+4²+……+99²)]÷2
=[(1+99)×99÷2+99×(1+99)×(2×99+1)÷6]÷2
=(100×99÷2+99×100×199÷6)÷2
=166650