1/2*5+1/5*8+1/8*11+1/11*14+.+1/17*20=?

问题描述:

1/2*5+1/5*8+1/8*11+1/11*14+.+1/17*20=?

原式=1/3(1/2-1/5+1/5-1/8+1/11-1/14+……+1/17-1/20)
=1/3(1/2-1/20)
=3/20