ok so let us quickly review on the few basic topics that we covered in earlier grades so let's just first start with what do we mean by a line segment so we know that a line segment has two endpoints the figure number one here shows a line segment and we have this point B and Q right so line segments simply is characterized by a line having two end points right and we denote this line as PQ right or you can even just write this or simply you can say BQ that's this the line segment PQ then let us look at line line is nothing but the concept of line segments extended further so in line segments there are no end point so if you see that we have these two points and they are two points and they are extending here this this line is extending infinitely in both the directions and this is what the arrows represent so that is what is a line segment and array is nothing but a line segment or a line which has one endpoint so in this case we have this row a raid 0 P and we represent array as opie followed by a single arrow so just shows array and if I want to represent the line a be really present a be as this showing the air on both the sides which shows that the line this is a line extending indefinitely in both the directions we also use a letter L mm to represent line so we can say this is a line L or a line m online n just for the sake of simplicity instead of writing this notation every time it is easier for us to use a single letter ok so that was about lying ray and line segment now let us quickly review water angles now if you remember angles are nothing but the corners and the corners are formed whenever two line segments or two lines meet so for example if look at this figure here which is that of a triangle and what do we see is we have three angles over here and how do we say that we have three angles because this segment and this segment they meet over here they meet at this point right so this we known as vertex if you remember so wherever there is a vertex there is an angle similarly this segment and this segment right they are meeting at this point or this vertex and we have angled so let's say this is an angle a oh let's say this is an angle a let's at this angle B and this angle C so we have three angles over here right now similarly if you look at this we have just two simple line segments okay and they are meeting at this point and we can you can see that it is forming angles this is one angle this is another angle let me write this angle one angle to angle three and angle for so simply angles are formed whenever we have two lines meeting at a point okay now let us recollect some basic types of angles so we learn that there is an acute angle and an Acura angle is an angle which is between 0 to 90 degrees now if you remember acute angle could be something like let's say that we have this and something like this so this is an acute angle we could even draw another angle here or we could draw this ray over here or here or here or we can keep on doing all the way long till you reach 90 degree the moment we have a 90 degree angle we call it as a right angle right angle is an angle which is equal to 90 degrees as is shown over here right and any angle that is less than 90 degrees that is still here till here right we can say that that angle is an acute angle okay then we come to an obtuse angle an obtuse angle is between 90 to 180 degrees that is when we start looking at the other part of the angle if you see here when we extend it beyond 90 degrees so the same ray I am drawing different angles if you can see that right so if I were to look at it in another way what we can do is something like this so we make a straight line and then we make an angle like this perpendicular right and if I want to now say which angles are which we can say that from here to here any angle that lies from here is an acute angle this angle which is like 90 degrees right this is a right angle and any angle that lies in this area that is an obtuse angle right so because this is 90 degrees this is 0 to 90 degrees and this is 90 to 180 degrees right so and that is an obtuse angle so these were some of the basic ideas of angles and lines now in the next session we'll start looking at more details about different types of related angles
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