Measuring Angles With A Protractor
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00:00Hi kids! Today we will learn how we can measure angles. So let's get started!
00:10We learned that angles are...
00:14Angle is a shape formed by two straight lines diverging from a common point.
00:22Here, B is the vertex of the angle.
00:27A, B and B, C are legs of the angle.
00:33Interior is the space between two straight lines.
00:40It extends even beyond the ends of line segments.
00:45Here, white area is the interior of the angle.
00:51Exterior is the space outside the two straight lines.
00:59It extends even beyond the ends of line segments.
01:05Here, the area marked with blue is the exterior of the angle.
01:12Now, let's learn types of angles.
01:19If the angle is less than 90 degree, we call it an acute angle.
01:27So, acute angles are less than one-fourth of a full circle.
01:34Now, let's learn about right angle.
01:40Exact 90 degree angle is called a right angle.
01:46So, we represent right angle like this.
01:52So, right angle is exactly one-fourth of a full circle.
02:00If the angle is more than 90 degree, but less than 180 degree, we will call it an obtuse angle.
02:12So, obtuse angles are more than one-fourth of a circle and less than half of a circle.
02:22Now, let's learn about straight angles.
02:28And 180 degree angle is called straight angle as it's a straight line.
02:38So, straight angle is exactly half of a circle.
02:44Now, let's learn what are reflex angles.
02:50180 degrees to 360 degree angles are called reflex angles.
03:00So, reflex angles are more than half of a circle and less than a full circle.
03:10Now, let's learn what is a full rotation or a paragon.
03:18360 degree is a full rotation or a full circle.
03:26Now, let's see how we can measure the angles.
03:32The amount of the turn from one arm to the other arm in an angle is called the size of the angle.
03:42And we measure the size of angles in degrees.
03:46Symbol for degree is this.
03:52Here we have an angle.
03:54Now, let's measure it.
03:58We use a protractor to measure the angles.
04:04It has two scales marked from 0 to 180 degrees.
04:14One is left to right and the other is right to left.
04:20We will see with examples why there are two scales on the protractor.
04:28The two scales make it easy for us to measure angles facing different ways.
04:34Here is an angle. We have to measure it.
04:40To measure the size of angle AOC, place the protractor over the angle so that the center of the protractor is directly over the angle's vertex that is O.
04:58And the baseline of the protractor is along the arm OA of the angle.
05:08We use the inner scale to measure the angle AOC as the arm AO passes through the zero of the inner scale.
05:20Following the inner scale around the protractor, we find that the other arm OC passes through the inner scale at 60 degrees.
05:32So the size of angle AOC is 60 degrees.
05:38We can write it like this.
05:42Now, we have another angle.
05:46We have to measure it using the same protractor.
05:52To measure the size of angle ABC, place the protractor over the angle so that the center of the protractor is directly over the angle's vertex that is B.
06:10And the baseline of the protractor is along the arm AB of the angle.
06:18We will use the outer scale to measure the angle ABC as the arm AB passes through the zero of the outer scale.
06:32Following the outer scale, we find that the other arm AB passes through the outer scale at 120 degrees.
06:44So, the size of angle ABC is 120 degrees.
06:52We write it like this.
06:56Now, let's measure another angle.
07:02To measure the size of angle CDE, place the protractor over the angle so that the center of the protractor is directly over the angle's vertex that is E here.
07:20And the baseline of the protractor is along the arm ED of the angle.
07:28We use the inner scale to measure the angle DEC as the arm DE passes through the zero of the inner scale.
07:42Following the inner scale around the protractor, we find that the other arm EC passes through the inner scale at 90 degrees.
07:56So, the size of angle DEC is 90 degrees.
08:04We write it like this.
08:08So, kids, we have learned how we can measure acute and obtuse angles.
08:17Now, let's learn how we can measure reflex angles.
08:23Now, we will use a protractor to measure the reflex angle ABC.
08:31To measure the reflex angle ABC, extend the arm CB to D to form angle CBD, which is a straight line or a straight angle.
08:49And we know straight angle measures 180 degrees.
08:55So, we will measure the size of angle DBA.
09:02Place the protractor upside down so that the center of the protractor is directly over the angle's vertex that is B here.
09:15And the baseline of the protractor is along the arm DB of the angle.
09:24We will use the outer scale to measure the angle DBA as the arm DB passes through the zero of the outer scale.
09:37Following the outer scale around the protractor, we find that the other arm BA passes through the outer scale at 45 degrees.
09:51So, the size of angle DBA is 45 degrees.
09:59We write it like this.
10:03To measure the reflex angle ABC.
10:08As the reflex angle ABC is equal to the sum of angle CBD and the angle DBA.
10:19180 degrees plus 45 degrees, we get 225 degrees.
10:30Now, let's learn what are complementary and supplementary angles.
10:37Two angles are said to be complementary angles if their sum is 90 degrees.
10:46Example, 40 degrees plus 50 degrees equals 90 degrees.
10:55So, 40 and 50 are complementary angles.
11:02Let's take another example.
11:0530 plus 60 equals 90.
11:10So, 30 and 60 are complementary angles.
11:16If we have one angle and we are asked to find its complementary angle, we subtract it from 90 degrees to get its complementary angle.
11:31Example, what is the complementary angle of 55 degree angle?
11:39For that, we will subtract 55 from 90.
11:4590 degrees minus 35 degrees, we get 55 degrees.
11:53So, 35 degree is complementary angle of 55 degrees.
12:00Now, let's learn what are supplementary angle.
12:07Two angles are said to be supplementary angles if their sum is 180 degrees.
12:16Let's take an example.
12:1970 plus 110 equals 180.
12:27So, 70 and 110 are supplementary angles.
12:35Let's take another example.
12:3850 plus 130 equals 180.
12:45So, they are supplementary angles.
12:49If we have an angle and we have to find its supplementary angle, we subtract it from 180 degrees.
13:00Let's take an example.
13:04What is a supplementary angle of 60 degrees?
13:09Kids, you have to subtract an angle from 180 degrees to find its supplementary angle.
13:19So, subtract 60 degrees from 180 degrees.
13:25We get 130 degrees.
13:29So, kids, today we learned how to compare angles.
13:35What are complementary and supplementary angles?
13:40Now, you may go ahead and take a quiz to learn more.
13:44Bye-bye.