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Constructing angles

Sal uses a protractor to create 10° and 155° angles. Created by Sal Khan.

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Video transcript

We're asked to construct a 10 degree angle. So we have this little angle tool here that we can use to construct an angle. So just like that. And then they give us a protractor to actually measure the angle. So let's set it up. Let's put the protractor here. Let's put the vertex of the angle at the center of the protractor, in other words, center the protractor at the center of the vertex. Looks like the protractor's a little bit easier to manipulate. So let's do that there. Now let's put one of the rays here at 0 degrees. And now I'm going to put the other ray at 10 degrees. And it looks like I am done. I have constructed a 10 degree angle. And you want to be careful here when you use this tool because the angle in question-- let me move the protractor show you what I'm talking about-- that this right over here is the angle that we're talking about. If I'd switched these two rays, the way the tool is set up, it might have interpreted the angle as this outer angle right over here. So be careful to look at which angle we're actually measuring. But this looks like I did the 10 degree angle. So let me check my answer. And just to be clear what I'm talking about. If you did the 10 degree angle like this, then it definitely would have marked it wrong even though the interior angle right over here or this angle right here might be 10 degrees. The way the tool is set up, you see from this circle, that it thinks that you're looking at this outer angle. So it's important to make sure, at least for the sake of this exercise, that the system, that the computer program knows which angle you're going to talk about. Let's do one more of these. 155 degree angle. And this one's interesting because this is an obtuse angle. So once again, let us put the protractor at the vertex of the angle. So just like that. And now that seems pretty good. Now let's take one ray and put it at 0 degrees, and then let's take the other one and put it at 155. So once again, 10, 20, 30, 40, 50, 60, 70, 80, 90-- that gets us to a right angle. Then we'll start getting into obtuse angles, 100, 110, 120, 130, 140, 150. And just to make sure that blue arc is measuring this angle right over here, not the outer one. And let me move the protractor out of the way so we can get a good look at it. And we got it wrong. So let's see what we-- oh, 155 degree angle, not 150 degree angle. Let me fix that. 155 degree angle. Now we got it right.