be busy+in还是on还是at还是不加

be busy in doing sth（正忙于做某事），一般将in省略了，就写成be busy doing sth了
没见过be busy on或者at的，只有be busy with sth，意思和be busy in doing sth一样，如：
1.be busy with sth
例如:I am busy with my homework!

2.be busy in doing sth
例如:I am busy in doing my homework!

你是问be busy doing sth.吗?

be busy后如果直接加名词的话用with连接

be busy in doing

be busy doing sth, 什么都不用加了！