Simple operation: (1 + 3 + 5 +...) +1999)-(2+4+6+… +How to do it?

Simple operation: (1 + 3 + 5 +...) +1999)-(2+4+6+… +How to do it?

（1+3+5+…… +1999）-（2+4+6+…… +1998）
=（1-2）+（3-4）+（5-6）+…… +（1997-1998）+1999
=-1-1-1-…… -1 (999 - 1) + 1999
=-999+1999
=1000
If you don't understand, please ask,

If "!" Is a mathematical operation symbol, and 1! =1，2！ =2×1，3！ =3×2×1，4！ =4×3×2×1… . find the value of 100! 98

∵1！ =1，2！ =2×1，3！ =3×2×1，4！ =4×3×2×1… ，∴100!98!=100×99×98×97×… ×198×97×96×… ×1=9900．

And "!" is an operation symbol, and 1! = 1, = 2x1, = 3x2x1, 4 = 4x3x2x1,..., then what is the result of 2012! / 2011?

=2012x2011!/2011!
=2012