demonstration that what is to be proved follows from the premises.
You should try to start by stating the premises, and what you are going to
You should finish by observing that what you have just done demonstrates
what was to be shown is in fact true.
How you get from the start to the finish is up to you, in a formal proof
environment you will have a strict structure and a set of rules for moving
from one line to the next. But most proofs are not like that, they are less
formal. In an informal proof you are at liberty to use anything that comes to
hand (as long as it is well established or already proven) and follow the
normal rules of reasoning.