4000-5-10-15-...-95-100=?

问题描述:

4000-5-10-15-...-95-100=?


4000-5-10-15-...-95-100
=4000-(5+10+15+...+95+100)
=4000-(5+100)X20/2
=4000-1050
=2950

先要算出5+10+15+20+.+100的和,再从4000里面减去.
5+10+15+.+100
=5+2x5+3x5+...+20x5
=5(1+2+3+...+20)
=5x(21x10)
=1050
4000-1050=2950