原式=1/3×（1/2-1/5）+1/3×（1/5-1/8）+1/3×（1/8-1/11）+...+1/3（1/20-1/23）=1/3×【1/2-1/5+1/5-1/8+1/8-1/11+...+1/20-1/23】=1/3×（1/2-1/23）=1/3×21/46=7/46
