Dear sir,

How are you recenty? Do you have some spare time? Our school is going to host an annual seminar on Corporate Socal Responsibilties in September. We are expecting you could come and be a keynote speaker at the seminar. If you are so kind and have the time to come, we'll appreciate it very much. So please reply this ltter as soon as possble to tell us whether we'll have the honor of listening to your keynote speech.
We really expect your reply.

Yours,
Student

题目要求以书信体格式写一封邀请函，具体书信格式可参考范文格式。范文开头点题，结尾不忘要求对方尽快回信，题目中并未明确说明时间地点，书信中可略去也可随意编写。