## Directions

You have just come back from the U.S. as a member of a Sino-American cultural exchange program. Write a letter to your American colleague to
1) Express your thanks for his/her warm reception;
2) Welcome him/her to visit China in due course.
Do not sign your own name at the end of the letter. Use “Zhang Wei” instead.

## 前言

$$\frac{A}{(x-aa)} + \frac{B}{(x-bb)} = \frac{cx + d}{ax^{2} + bx + c}$$