# What is a Theorem?

Reference > Mathematics > Introduction to ProofsSo far we've talked mainly about algebraic properties, like "The additive property of equality," which says that if you have a true equation, you can add the same thing to both sides and it will still be a true equation. Now, we never *proved* that this property is true; we just accepted it as being true. In geometry, things that you just accept to be true without proving them are called "axioms" or "postulates." So when you see "axiom" in a geometry class, or "postulate," you can just think to yourself, "This is like those algebra properties that we accepted as true, without proving."

But when you take a geometry class, you won't just talk about axioms and postulates; you'll also use *theorems*. A theorem is anything that you've proved, that you think is useful enough that you might want to use it again sometime. Let me show you what I mean. Here's a proof for us to do:

STATEMENT REASON 1. ax + b = 0 Given 2. a ≠ 0 Given 3. ax + b - b = 0 - b Subtraction property of equality 4. ax = -b Combining like terms 5. ∴ x = -Division property of equality (divide by a; 2, 4)ba

Note that I was required to specify that a ≠ 0, because the division property of equality requires that we not divide by zero. Therefore, I had to include that step as part of my reason for using the division property. Also, notice that I shortened my proof by clarifying in step 5 what I divided by, which allowed me to avoid having an extra step where I *showed* what I divided by, before showing the result.

At this point, all I have is just another proof, but as I consider this problem, I realize that it's very common to have to solve a linear equation in the form ax + b = 0, and it would be nice if I didn't have to do those 5 steps *every single time I have to solve an equation like that.* So I decide that I'm going to give this proof a name, and then I will be able to use this proof as a *reason* in another proof.

What should I call this proof? Hmmm...how about "Mr. T's Linear Equation Theorem"? That's got a nice ring to it, don't you think? And just by giving the proof a name, it is no longer just a proof - it's a theorem!

So let's take a look at how we can use this in another problem:

Prove: If 2x + 6 = 0, then x = -3

We've done problems like this before, and they took us several steps to prove. Now, though, our proof is going to be*very short*, because I have theorem that does most of the work for us:

STATEMENT REASON 1. 2x + 6 = 0 Given 2. x = -Mr. T's Linear Equation Theorem 3. ∴ x = -3 Arithmetic62

Wow! Wasn't that easy? It's a whole lot nicer than using the extra steps of subtraction property and division property. Using a theorem can make my proofs a lot shorter! In the next section we'll create another theorem, and then we'll make use of the new theorem, along with this one, to make a long proof very short!