A={1,3,5,7} B{1,4,7} C={2,5,7,8,}
问题描述：
A={1,3,5,7} B{1,4,7} C={2,5,7,8,}
A↓B{1,3,5,7}↓{1,4,7}={1,7}
A↑B={1,3,5,7}↑{1,4,7}={1,3,4,5,7}
特殊符号↑ ↓
｛A↑C}↓B=?
答
｛1,2,3,5,7,8}↓｛1,4,7}={1,7}