形容词Playing ------ is the most important principle in a compe

A
首先这两个词在句中都是副词
play fair 是一个固定用法吧..=behave honestly
fair可作形容词,名词和副词
做副词的意思:according to the rules;in a way that is considered to be acceptable and appropriate
fairly是副词,有两个意思,一个是相当地,一定地;另一个是in a fair and reasonable way;honestly..
这样看fair的副词义和fairly的第二义很接近
但体会例句:
Come on ,you two,fight fair!
He has always treated me fairly.
可以体会到,fair应该指在一种规则的层面表示公平,fairly在个人的层面表示待人公平...