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Measuring angles using a protractor

CCSS Math: 4.MD.C.6

Video transcript

What I have right in front of me is the Khan Academy measuring angles exercise. I have a small part of it in this screen right over here. And it's a pretty cool exercise because it has this little virtual protractor that we can use to actually measure angles. And I want to give credit to the person who built this protractor, because I think it's pretty neat. Omar Rizwan, who was actually a high school intern, made this pretty neat module. And so in general, when you want to measure an angle, what you want to do is you want to put the center of your protractor at the center of-- or I should say, at the vertex of the angle. Or you want to put the vertex of the angle at the center of the protractor. And then what you want to do is either rotate the angle or rotate the protractor. In this case, we're going to rotate the protractor. You want to rotate the protractor so that the 0 angle, or kind of the 0 mark, is at one of the sides of the angle. And the other side of the angle is within the protractor. So let's try to do that. So maybe if we want to do that, this 0 side should be at this side of the angle. So let's rotate it that way. Let me just keep rotating it. If I could just keep it pressed. That's better. All right. That looks about right. So one side is at the 0 mark. And then my angle, my other side-- or if this was a ray, it points to, looks like, pretty close to the 20 degree mark. So I will type that in off the screen. You don't see that. And that is the right answer. And then we can get another angle. So let's try to measure this one right over here. So once again, place the center of the protractor at the center, at the vertex, of our angle. We can place the 0 degree, the base of the protractor, at this side of the angle. So let's just rotate it a little bit, maybe one more time. That looks about right. And then the angle is now opening up-- let's see, the other side is pointing to 110 degrees. So this is larger than 90 degrees. It's also an obtuse angle. The last one was an acute angle. This is obtuse, 110 degrees. More than 90 degrees. So let me type it in. I got the right answer. Let's do a couple more of these. So once again, put the center of the protractor at the vertex of our angle. And now, I want to rotate it. There we go. And this looks like roughly an 80 degree angle, not quite. If I have to be really precise, it looks like it's maybe 81 or 82 degrees. But I'll just go with 80 as my best guess. I got the right answer. Let's do one more of these. So once again, vertex of my angle at the center of my protractor. And then I want to put one side of the angle at the 0 degree. And I want to show you, there's two ways to do that. You could do this. You could do just this. But this isn't too helpful, because the angle is now outside. The other side sits outside of the protractor. So you want the 0 degrees on the side, so the other side is within the protractor. So let's keep rotating it. There we go. And then our other side opens up or you could say points to 70 degrees. So this is an acute angle right over here. So it is 70 degrees. So I'll leave you with that. Oh look, I'm ready to move on, the exercise tells me. And now we can start talking about more things about angles now that we know how to measure them.