 Hello and welcome to another session on geometry and we discussed in the previous session that we are going to discuss triangles and we are going to deal with its properties We have been just dealing with triangles all our life, you know You know, whether it is a sandwich bread or whether it is a snack or a pyramid or you know, you know, simple in the traffic You you deal with triangles every time. Yeah, it's very common geometric shapes in childhood We have been studying about it. Now is the time when we will be Finding out a little bit more about it and learning its properties Now as the name suggests try angles The word can be broken down into two words try and angles angles you already studied in the previous sessions. So now How do we define triangles then so triangles are nothing but polygons first of all polygons the most elementary polygon you can You know, imagine a polygon. What is a polygon poly means many gone means side. So, you know, hexagon, heptagon, decagon Triangles, square, quadrilateral, all these are polygons, right? So, you know Closed figure. So what is polygon? Call you polygon is a closed plane figure Okay, planar Figure right that means the figure must be drawn on a Plane that is for example while you are doing it on a piece of paper on your notebook So it must be the entire shape should be on the plane on the same plane, right? So you will be asking can there be any other geometric shape, which is not totally into one plane? Yes, a sphere a cuboid you cannot imagine a cuboid over a plane, right? so a polygon what is a polygon polygon is a closed planar figure and Depending on the number of sides we have First of all triangles then quadrilateral then pentagon then hexagon and things like that so triangle is a polygon, right and Polygon with three sides with Three sides, right? So the minimum number of sides to frame a polygon is Three so you can't have a polygon with two sides, right? So these two sides You know, so we basically you can't close you can have a you can't have a closed figure using a using only two lines Right, so hence that's the constraint that's the constraint with two lines Is it it so you can? However have three lines Right three lines joined together to make a polygon Correct, right. So this is what is a polygon Right, and this is actually a triangle now you can imagine, you know polygon with four sides This is a quadrilateral then you can have pentagon Okay, so you can have pentagon also. So one time will be five sides correct. So this is Pentagon right so Likewise, you can have hexagon with six sides So this is pentagon. Sorry. So with six sides you can have hexagon like that So you can have any number of You know or different types of polygon with any number of sides But the side minimum number is three. So we are going to study the first elementary polygon that is the Triangles, right? So that's what is the scope of this series. Let me just remove them Yeah, so what did we learn triangle polygon a close plane figure with three sides? Okay, that's good enough Definition so what all characteristics or you know, let's say parts of triangle are and how do we name them? So we use English alphabet letters to Name them. So for example, this triangle can be named as Triangle we make this Greek letter delta sign Okay, Delta Delta is for triangle So Delta looks like a triangle isn't it? So what is the Delta basically Delta is nothing but Geographic feature the mouth of the river where it meets the Ocean or the sea right? So the water actually gets distributed and the top view looks like a triangle So Delta right since Greek letter Delta and then we write the three letters ABC like that So this is the triangle ABC Triangle so name of triangle becomes triangle ABC Okay, now a B and C These are called vertices Vertices is plural of vertex vertex Okay, so ABC are vertices then a b BC and C a are sides edges Right, so either they are slides or edges of triangle ABC, right? And then we have three angles This one this one this one. So we have angle ABC Then angle BC a and then angle C a b these are three Angle so we see This is what are the let's a feature of triangle many a times We also name or name them using some numerals like that. So you can say angle 1 angle 2 angle 3 okay, now Anything inside the bound figure this is called interior interior of the triangle Right, so if you see this is called interior and everything outside obviously will become This side this side this side. This is called exterior of the triangle Exterior of the triangle Isn't it so what do we learn we learn that In triangle, what is a triangle triangle is a polygon so closed planar figure closed means, you know It has boundary so it divides the plane into two parts or other three parts the triangle itself can be another part So interior exterior and the triangle itself then it is bound by three sides and it is denoted by a delta symbol and followed by the three Letters ABC there is no need to follow one particular order in naming So the same triangle triangle ABC is as good as This is same triangle BC a so you can have in any order The naming you can name the triangle in any order and then a C a B Right in any order you can name it no problem, right now