忙于做某事 be busy (in) doing sth
忙于某事 be busy with sth
两者相等,例如
I am busy doing my homework now = I am busy with my homework now.
我现在忙着做作业
忙于做某事 be busy (in) doing sth
忙于某事 be busy with sth
两者相等,例如
I am busy doing my homework now = I am busy with my homework now.
我现在忙着做作业