In the past few minutes we've touched on a lot of concepts that are going to play a really central role in the rest of the course and really in all math going forwards from here. So I would like to do a review of the algebra vocabulary that we've covered so far. I think it's actually going to amaze you how much we've already gone over in a pretty short amount of time. So first thing first, we have numbers, as one might expect in math. So what are some examples of numbers? Well, we could have any number of things. So like we learned in the very beginning of this course, they're all different kinds of numbers. Some that are pretty simple and we use all the time, like 0 or 1 or 46, and some that are a little bit more special, like the root of 2 or pi. Then in contrast to numbers, we also learned about variables. So, here we have all different kinds of symbols, we have some letters, we have some sillier things like hearts and smiley faces, we got some greek letters. But regardless of what we write variables as they all have the same purpose, as we learned earlier. Variables are symbols that we use in equations and expressions that serve as slots into which we can plug other numbers to make our expressions take on different values. So comparing numbers and variables, numbers have fixed values. They inherently show certain quantities. Variables, however, don't show outright what they stand for, they're just place holders into which we can insert different numbers. One thing to take note of when we're looking at what's inside each of these circles is that we actually have a symbol on the number side. That's this pi right here. Since pi is an irrational number, it extends for an infinite number of decimal points. Now, it took me an awfully long time to write all of this out. So if I were to write even more, that would take me even longer. To make our lives much more simple, we give this infinitely long number a special name, which is pi. A similar thing happens when we write square roots. We have a number, 2, right here but we also have a square root sign over it. Since the root of 2 is also an irrational number the square root sign is just another way of taking a shortcut to not have to write out an infinite number of decimals. So bottom line, when you see symbols, they're almost always variables. They'll be certain cases. Which you'll become well acquainted with when you see things like pi or like a square root sign that are symbols of some sort but represent numbers.