0.91*99+0.99
=91*0.99+0.99
=（91+9）*0.99
=100*0.99
=99
3-5+7-9+11-13...+1995-1997+1999
=（ 3-5）+（7-9）+（11-13）...+（1995-1997）+1999
=1999-2-2-2-……-2
=1999-1998
=1