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=?
答
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