Sometimes one member of a group will fall behind because he's not famliar with the solution methods that are being discussed, or some of the terminology being used:

A: we know that cosine x has to be at least 1/2.
B: what's cosine?

This doesn't necessarily mean that he shouldn't be part of the group. Everyone knows about some things, and not about others.

While you usually don't have time to give an entire introduction to an unfamiliar subject, sometimes you can give enough information to help him follow the general outline of the conversation, e.g., give a quick definition of the sine and cosine functions, or a quick explanation of similar triangles.