 Hello and welcome to another session on geometry. So with today's video lecture we are going to start a series on special shape in geometry that is called triangle. Now all of you are familiar with this geometric shape since childhood when you start your mathematics journey. So in the first geometry lessons you would have studied about line, you studied about different shapes like triangle, square and all that. So now is the time when we are going to delve a little bit deeper into the characteristic of this special geometric shape that is called a triangle. Okay so I have displayed a triangle over here. So what is a triangle guys? So as we have defined every other concept in mathematics we will be defining triangle as well. So triangle first of all is a plane figure. Okay so plane figure but what kind of plane figure you can see you can observe that there are three edges to this shape three edges are very very prominent and now that you also have a knowledge of angles you can also see that there are three angles right three angles in this particular shape also two edges are meeting at one point and there are three such points and we have given a name to such point of intersection of the edges we call them vertex and together all three are called vertices okay so vertex there are three vertices in the triangle there are three sides in the triangle there are three angles in the triangle so hence it justifies the name try angles you can see try which means or which stands for three and angles you know whenever two lines intersect the region is divided into four parts basically and each of the region each of the four parts that is when two lines intersect form an angle so we have discussed this in the previous sessions as well now triangles have a long history guys and if you see around you there are lots of triangles in our day-to-day lives as well so if you see traffic signals most of the time if you will see the the communication which you know the traffic release does or the road signs which are there are triangular in shape so you eat lots of snack which are triangle in shape so in sandwiches if you have seen that's triangle in shape most of times so hence you know lots of triangles are around us but in these sessions what we are going to do is we are going to understand their properties and it's not that we are doing it for the first time triangles are being studied since a long long time so Greeks Indians Chinese Egyptians all of these people have been studying triangles and you know about a person famous mathematician called Euclid the first four volumes or you know so you know the the book elements written by Euclid in some third century BC it has 13 volumes and the first four volumes actually deal only with triangles and its properties so the angles some property or the exterior angle property which we are going to see all of them all of them were codified by this scholar or mathematician called Euclid around third century BC and we are studying all those properties since then till now so triangles are very useful so if you've seen how much you know if you have seen lots of triangles over there in monuments in building making in let's say for example Egyptian pyramids if you see there are lots of triangles so in architecture construction triangles are used then those who are into video games or you know graphics so they know what the importance of triangles over there so triangles as a subject matter will be very very interesting if properties are immense very very interesting so in these subsequent sessions what we are going to do is we are going to study all these properties one by one now we will be starting with classification of triangles where in how do we classify triangles and what's the use of that classification we'll be taking up in the next session so I welcome you all in this series of sessions which we will be devoting to study towards or study of triangles so this is just a small demonstration of moving triangle which I have written in your demonstrated in GeoGibra software so you wish you can download the software we will be using the software a lot to understand different properties of triangles going forward so let's begin and let's start first with defining triangle and then classifying them