1、I spent half an hour（working on）this difficult math problem 括号里为何填working on 而不是to wo
问题描述：
1、I spent half an hour（working on）this difficult math problem 括号里为何填working on 而不是to wo
答
spend doing....
答
spend 后可以跟不定式，也可以跟动名词，所以跟working on是可以，当然也可以填写to ....
答
spend 有两个用法：spend some time （in）doing sth.spend some time on sth.
It takes sb some time to do sth.
上面的区别就可以很清楚说明如何填这个空了.