I think I did not explain myself well when posting this.
The problem is to prove the equality. But no equations are necessary. For example, in a previous exercise it's asked to prove that is the identity in the operation #. It sufices to say that a sphere without a part that is homeomorphic to an open disk is homeomorphic to a closed disk.
If that's not the problem, could you tell me if this problem is a hard one for a 3rd year student of mathematics?