1/7+5/6+(-4/7)+(-1/2)＝1/7+5/6-4/7-1/2＝(5/6-1/2)+(1/7-4/7)＝(5/6-3/6)-(4/7-1/7)＝1/3-3/7＝7/21-9/21＝-2/21
