(1/11+1/20+1/31+1/40）*（1/21+1/30+1/41+1/50)-(1/11+1/21+1/31+

题目错了吧?
题目是不是这样的：
(1/11+1/21+1/31+1/41）*（1/21+1/31+1/41+1/51)-(1/11+1/21+1/31+1/41+1/51）*（1/21+1/31+1/41)
令a=1/21+1/31+1/41
∴原式=（1/11+a)*(a+1/51)-(1/11+a+1/51)*a
=a/11+a²+1/11*51+a/51-a/11-a²-a/51
=1/561