reciprocal variational problems

例句：

1. 1. The two companies have a reciprocal agreement on their products.
   这两家公司拥有关于产品的互惠协议。
2. 2.The teacher and the student are reciprocal friends.
   老师和学生是互相的朋友。
3. 3.The reciprocal of a number is the product of its reciprocal and itself.
   一个数的倒数是它的倒数乘以它自己。