 Hi, I'm Zor. Welcome to Unizor Education. We will continue talking about basic elements of geometry, only definitions, no more than that in terminology, and today we will talk about circles. Well, again, everybody knows more or less what circle is, so we will just introduce a couple of maybe definition and introduce some terminology. So if you have a circle, basically you can describe it as a set of points on the plane, which are located at the same distance from one fixed point, which is called the center. So one particular point is called the center, everything else around it, on this particular curve are the points which are located on the same fixed distance from this center point. Now, any segment which is drawn from the center to one of the points on that curve, which is called the circle, is called radius. Now, any segment which connects two points on the circle and contains the center is called a diameter. Any segment which connects two points on the circle without the property of containing the center is called a chord. So basically diameter is a chord which has an additional property of also containing the center as well. So this is also a chord and this is a chord. But these are all chords. And the chords which contain the center are diameters. So this is diameter and this is diameter. So they're all diameters. All right, I put so many lines that we can't really see anything else about it. All right, so let's do it again. Now, what else do we have? We have a chord and we have, let's say, two radius. There is certain duality in the term circle. Sometimes it's a curve, sometimes it's error within the curve. Most likely we will consider just the curve when we are talking about circles. And if you want to talk about the whole area within, we will probably say the area within the circle. Now, as far as areas are concerned, you can actually consider this particular piece of the area within the circle, which is bounded by a chord and a piece of a circle. Now, by the way, this piece of a circle is called arc. So that's one thing and this is another thing, another piece which can be considered. So they also have their names. One of them is called, I think, circular segment, or this is the segment, rather. And this looks like a pizza, actually. Well, anyway, these are different pieces of a circle or the area within the circle, which we will consider during our discussions with certain properties, theorems, problems, maybe, etc. So let me just summarize what we know. Circle, center, radius, chord, arc, diameter, or area within the circle, which might actually be bounded by a certain, either chord or two radiuses. So that's basically it. And it's just a terminological introduction into the concept of a circle. By the way, circle is something which is very thoroughly started by Greek mathematician Euclid, I mentioned before as the author of five famous axioms. He was one of the first mathematicians who wanted to measure the lengths of this circle. And basically, at that time, his research was quite advanced, actually, considering that he didn't really have any predecessors, etc. So anyway, so these are major components of a circle, which we will be considering during our future lectures and the theoretical and practical study. Thank you very much.