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Recognizing angles

Sal identifies acute, obtuse, and right angles in diagrams and pictures. Created by Sal Khan.

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Video transcript

Look at the diagram below, then answer the following question. What type of angle is the angle highlighted in the green below? So it looks like two anteaters going after an ant right over here. And this anteater is going straight down, or at least on my screen it looks like it's going straight down. And this anteater's going straight to the left. And so it looks like this one's going straight down, this one's going straight to left. It looks like this is a right angle or a 90-degree angle. And so if I have to pick between any of these, it looks most like a right angle. It looks like this is exactly going down, and this is going exactly to the left, that this is a 90-degree angle here. If this was an acute angle, then the angle between them would be smaller. So maybe this top one would be coming in from this direction right over here. If it was obtuse, this angle would be larger. So let's just check our answer and try to do a few more of these. Which of the angles labeled in the diagram below are acute angles? So just as a reminder, an acute angle is less than 90 degrees. So let's see, A and C both look larger than 90 degrees. But B and D both look less than 90 degrees. So I'll go with B and D are the acute angles here. Let's do another one of these. What type of angle is the angle highlighted in green below? Well, this angle is less than 90 degrees. If it was equal to 90 degrees, this thing would come out like this. So this is clearly less than 90 degrees. This is an acute angle. Let me just do one more. This is a lot of fun. Select a symbol from the drop-down menu to complete the following statement. Then answer the question below. Angle A is less than 90 degrees. What type of angle is angle A? So angle A, it's less than 90 degrees. So an angle less than 90 degrees is an acute angle. If it was exactly 90 degrees, it would be a right angle. If it was larger than 90 degrees, it would be an obtuse angle. So I will go with acute angle. And we got it right.