1）1/1×6+1/6×11+1/11×16+1/16×21
问题描述：
1）1/1×6+1/6×11+1/11×16+1/16×21
2）1/8+1/24+1/48+1/80+1/120
3)(1/2+1/3+1/2004)(1+1/2+.+1/2003)-(1+1/2+.1/2014)(1/2+1/3+.+1/2003)
答
1）1/1×6+1/6×11+1/11×16+1/16×21=1/5(5/1×6+5/6×11+5/11×16+5/16×21)=1/5×(1-1/6+1/6-1/11+1/11-1/16+1/16-1/21)=1/5×(1-1/21)=1/5×20/21=4/212）1/8+1/24+1/48+1/80+1/120=1/4(1/2+1/6+1/12+1/20+1/50)...