(17/31+37/53+39/91)×（37/53+39/91+31/51)—（17/31+37/53+39/91+31/51）×(37/53+39/91)
问题描述：
(17/31+37/53+39/91)×（37/53+39/91+31/51)—（17/31+37/53+39/91+31/51）×(37/53+39/91)
答
设17/31=a 37/53=b 39/91=c 31/51=d
设b+c=e
原式=(a+b+c)(b+c+d)-(a+b+c+d)(b+c)
=（a+e）(e+d)-(a+e+d)e
=ae+ee+ad+ed-ae-ee-de=ad=17/31 x 31/51=17/51