999.3-998.2+997.3-996.2+...+3.3-2.2+1.3-0.2
=（999.3-998.2）+（997.3-996.2）+……+（3.3-2.2）+（1.3-0.2）
=1.1+1.1+……+1.1+1.1
=1.1×500
=550