a+2a+3a+4a.+100a
=100a+1+2+3+.+100
=100a+（1+100）*100/2
=100a+101*100/2
=100a+5050
当a=-2时
100a+5050
=100*（-2）+5050
=-200+5050
=4850
我这块学的最好了