In the last lesson, you learned about benchmark angles like 90°, 180°, and 270°.
These angles are easy to recognize.
This angle measures 90°.
It's a right angle.
Now, what's the measurement of this angle?
Yes, it's 180°.
We know that a straight angle is 180°.
How about this one?
This one's a bit tricky.
It's not 90°. But it's also not 45°.
It looks greater than 45°.
How can we measure this angle?
We can use a protractor.
A protractor is a tool used to measure angles.
It usually has two sets of numbers that go in opposite directions. They are written in different colors to help us easily tell them apart.
The numbers in our protractor are in black and blue.
Let's measure this angle using a protractor.
First, put the vertex at the center of the protractor.
The vertex is the point where the two arms of the angle meet.
Then, measure the size of the opening that the two arms form.
Tip: To get the correct measurement, make sure that one arm is pointing at 0.
Note that we're looking at the blue numbers.
What number is the other arm pointing at?
Correct, at 60.
We express the size in degrees.
What is the measurement of this angle?
That's right. It's 60°. ✅
Let's try to measure this angle with a protractor.
What's the first step?
Correct. Position the protractor so the vertex at the center.
Should we look at the black or blue numbers?
Be careful which one you choose.
When unsure, ask yourself this:
Is this angle is bigger or smaller than 90°?
Our angle looks less than 90°, so it cannot be 150°.
This means that we look at the black numbers.
Measure the distance between the two arms.
What's the angle measurement?
Very good. It's 30°.
Let's measure this obtuse angle.
First, we position the vertex at the center of the protractor.
Which numbers should we look at? Blue or black? 🧐
Here's a trick:
1. Find the arm that's pointing at zero.
2. Look at the color of the zero that this arm is pointing at.
3. That's the color of the numbers that you should be looking at.
So which numbers should we look at?
Yes. The blue numbers.
Now, let's measure the angle.
What number is the second arm pointing at?
Remember: We're looking at the blue number.
Very good, It's 140°. ✅
What if our angle looked like this on a protractor?
Because neither of the arms is pointing at zero, we can do subtraction.
The left arm is pointing at 130.
The right arm is pointing at 10.
Remember: Look at the same set of numbers. In this example, we're looking at the blue numbers.
Let's subtract:
130 - 10 = 120
The measurement of this angle is 120°. ✅
Tip: You could also have rotated the protractor until one of the arms in the angle was parallel to the side of the protractor.
Good job learning how to use a protractor.
Now, complete the practice to master using a protractor. No actually protractor is needed.