# find formal proof for a simple tautology

Could you please help me figure out the formal proof for the following argument? This is an example from the textbook "Language, proof and logic" (by D. Barker-Plummer et al). I am doing it in the program called Fitch.

$$\begin{array}{r} A\land (B\lor C)\\ \hline (A\land B) \lor (A\land C) \end{array}$$

The answer is obvious, if you formulate an informal proof, but I can't get it right formally. The screenshot below shows exactly where I've got stuck...

Any hints will be greatly appreciated! Thank you!

-
This is now the third of these questions that you're posting. a) It would be good style to link them to each other so people can build on the effort that's already been put into figuring out what formal system you're using etc. b) Three very specific very similar homework questions seems a bit much -- if the answers to the other two haven't helped you with this one, perhaps you should try to figure out on a more general level which piece of insight you're missing for doing these? –  joriki Dec 15 '11 at 0:15
What rules are we using? –  simplicity Dec 15 '11 at 0:35
I think you refer to this text, but I'm not sure it's the same edition ssdi.di.fct.unl.pt/~pb/cadeiras/lc/0102/lpl%20textbook.pdf Note 5. isn't correct, because you haven't derived "C". –  Doug Spoonwood Dec 15 '11 at 2:53
And after four question you still have not disclosed which rules you're using. Are you reading the comments at all? –  Henning Makholm Dec 15 '11 at 3:55