4000-5-10-15-…-95-100=?

4000-5-10-15-…-95-100=?

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