In this video, let's discuss how to measure angles.
Now, over here you have a point.
Imagine that this point over here right now, one complete revolution around this point is divided into
three equal parts and each part we call a degree.
So if you move from here and keep rotating your like this, like this, like this and keep doing till
you get back to here, we say that you have moved 360 degrees.
All right.
Let's take this as our reference line and we move in this direction.
All right.
Now we keep moving till we reach over here.
Now you can see that we have formed 90 degrees right here.
You can see we have 90.
So we have taken up 90 out of 360 degrees to reach still here.
But this type of an angle is 90 degrees and we call a 90 degree angle as a right angle.
But this is a very important concept in geometry.
Now, let's move on now say you keep rotating, all right.
From here, you keep rotating in this direction and you reach over here.
Now you can see that you have reached 180 degrees with this type of an angle is a 180 degree angle.
And we call such an angle as a straight angle.
All right, now say you keep rotating and you reach over here to this angle is 270 degree, right?
Because you can see that this is 270 over here and you keep rotating till you reach over here itself.
And that type of an angle is called a complete angle or it's 360 degrees because we have divided one
complete revolution into 360 equal parts.
And that's how we measure an angle.
So if you have an angle like this, you must remember in your school days you would use a protractor
and you can measure how many degrees this angle over here is.
So this could be 30 degrees or it could be any measure.
All right, let's move on.
Now we know how to measure an angle.
We have learned what is right angles.
We have learned what a straight angle is and what a complete angle is.
Let's move on.
Let's see what we mean with acute right obtuse and reflex angles.
We already know what right angles.
That's a 90 degree angle right now.
Let's move on.
What is an acute angle?
Say you have this angle over here right now.
Over here, you can see that if this area over here was at this point right, that would have given
us a 90 degree angle.
Right.
So definitely this angle over here is less than 90 degrees.
Right.
So that type of an angle is what we call an acute angle.
And acute angle is in between zero and ninety degrees.
All right.
Now we know what the right angle is.
That's a 90 degree angle, right?
That's what we call her.
Right angle.
And what is an obtuse angle.
Say you have an angle over here.
Over here.
If this right over here was over here, then it would have been a 90 degree angle.
Right.
So this angle over here is definitely greater than 90 degrees.
And that is what we call an obtuse angle.
And obtuse angle is between 90 degrees and 180 degree.
And remember, over here, 090 are not included.
Similarly, over here, 90 and 180 are not included.
Right.
Because the sign over here is just less than so.
X has to be in between zero and ninety four, the angle to be an acute angle and the angle has to be
between 90 and 180 degrees or to be an obtuse angle.
All right, let's move on.
What is a reflex angle that reflects angle is an angle which is greater than 180 degrees, but it has
to be greater than 180 degrees.
Now in these cases, we have seen an acute angle, right angle and an abuse angle.
At the same time, you also see that it reflects reflex angle is formed, right?
This angle, this angle and this angle.
You can see this is greater than 180 degrees.
Let's take an example.
Let this angle be thirty degrees.
We know that one complete revolution is 360 degree right there.
Therefore, this angle over here will be 360 minus 30, which is equal to three thirty degree.
What about this case?
We have ninety degrees over here.
Therefore, this angle over here will be 360 minus ninety, which is equal to 270 degrees.
What about this angle over here?
We know that this is an obtuse angle.
Let's take an example.
Let it be a hundred and ten degrees.
In that case, this angle over here will be 360, minus one hundred and ten, which is equal to 250
degrees.
What we have seen, what an acute angle is, what an right angle is, what an obtuse angle is and what
it reflects angles.
Remember, a reflex angle is greater than 180 degrees.