设a=1-1/2-1/3-1/4-1/5 b=1/2+1/3+1/4+1/5（1-1/2-1/3-1/4-1/5）x(1/2+1/3+1/4+1/5+1/6) - (1-1/2-1/3-1/4-1/5-1/6)x(1/2+1/3+1/4+1/5)=ax（b+1/6）-(a-1/6)xb=ab+1/6a-ab+1/6b=1/6x(a+b)=1/6x(1-1/2-1/3-1/4-1/5+1/2+1/3+1/4+1/5)=1/6x1=1/6