 Hi and welcome to the session. Today we will learn about measure of angle. Now first of all, measure of an angle is known as the degree measure of that angle. One complete revolution is divided into 360 equal parts. Each part is a degree so we say that one part is equal to one degree. One complete revolution means a complete angle. So a complete angle is divided into 360 equal parts and one part is equal to one degree. That means complete angle is equal to 360 degrees and we write it as 360 degrees. Now we have a straight angle and a straight angle is half revolution. That means half of 360 degrees so this will be equal to 180 degrees. Now we have a right angle and a right angle means one fourth revolution. That is one fourth of 360 degrees which is equal to 90 degrees. Now a straight angle is equal to 180 degrees which can also be written as 90 degrees plus 90 degrees. Now 90 degrees is a right angle so that means straight angle is equal to two right angles. Also one complete angle is equal to 360 degrees which can be written as 180 degrees plus 180 degrees. Now 180 degrees means straight angle so that means complete angle is equal to two straight angles. Also one straight angle is equal to two right angles so two straight angles will be equal to four right angles. That means complete angle is equal to four right angles. So in this way we can find out the relations between these angles. Now let's see how to measure an angle with the help of a protractor. Now you must be having a protractor in your instrument box. The curved edge of protractor is divided into 180 equal parts and each part is equal to one degree. So that means the curved edge of the protractor is equal to 180 degrees. Now the markings start from zero degrees on the right side and ends with 180 degrees on left side. And vice versa. Suppose we want to measure angle ABC. For this take your protractor and place it so that the midpoint of its straight edge lies on the vertex V of the angle ABC. Now adjust the protractor such that the line segment BC is along the straight edge of the protractor. Now if you will look at the protractor carefully then you will notice that there are two scales on the protractor. And for this angle you need to read that scale which has zero degree mark coinciding with the straight edge that is with the line segment BC. Now the mark shown by the line segment BA on the curved edge of the protractor gives the degree measure of the angle. Now on your protractor the line segment BA will show the mark of 60 degrees so that means angle ABC is equal to 60 degrees. We write it as measure of angle ABC is equal to 60 degrees or simply we write angle ABC is equal to 60 degrees. Now let's move on to our next topic perpendicular lines. Now two intersecting lines are perpendicular if the angle between them is 90 degrees. Now here AB and CD are intersecting lines and the angle between them is 90 degrees that is a right angle. So we can say that AB is perpendicular to CD and we write it as AB is perpendicular to CD. Now if AB is perpendicular to CD then we can say that CD is also perpendicular to AB that is AB and CD are perpendicular lines. Now let's see what is a perpendicular bisector. The perpendicular bisector of a line segment is a perpendicular to the line segment that divides it into two equal parts. Now here CD is perpendicular to the line segment AB also CD divides the line segment AB into equal parts that is if this point is O then AO is equal to OB. So for this we can say that CD is the perpendicular bisector of line segment AB. Thus in this session we have learnt measure of an angle, how to measure an angle with the help of a protractor, perpendicular lines and perpendicular bisector. With this we finish this session hope you must have understood all the concepts. Goodbye take care and have a nice day.