设：a=1+1/2+1/3+1/4,b=1/2+1/3+1/4.则原式=ax(b+1/5)-(a+1/5)xb=ab+1/5a-ab-1/5b
=1/5a-1/5b=1/5x(a-b).将a,b的值代人得：1/5x(1+1/2+1/3+1/4-1/2-1/3-1/4)=1/5x1=1/5.