How to make emacs switch input method automatically ?

[waterloo]

I use chinese to input normal text in latex .
And in math mode we almost use english .

I use `shift' to switch between chinese and english .

So now I use shift too frequently , my shift key is not sensitive .

How to make emacs switch  automatically ?

When  in math mode ,  emacs switchs to english .
When  in normal text ,emacs switchs to chinese. 

Can here anyone  give some instructions of doing this ?

I know there is function `texmathp' in auctex which determines if point is inside latex mode .

Thanks