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Course: Geometry (FL B.E.S.T.)>Unit 8

Lesson 3: Arc length (from degrees)

Arc length from subtended angle

Finding the length of an arc using the degree of the angle subtended by the arc and the perimeter of the circle. Created by Sal Khan.

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• I am confused about 18 pi as well. Aren't all circles 2 pi? Does 18 pi mean he is going around the circle 9 times?
• 2 pi radians is 360 degrees, so yes, all circles have an angle of 2 pi. In this video however, Sal is talking about the length of the circumference or a fraction of the circumference. This value will depend on the size of the circle.
• What does subtends mean
• If an arc subtends a particular angle at the centre of the circle, it means that if you draw straight lines from each end of the arc to the centre of the circle, that's the angle you get between the two straight lines.
• I don't understand where 18 pi came from?
• Hi Sara. 18π is the circumference of the circle and was given to us in the problem statement.
• How would you solve this problem if you were given the radius of the circle instead of the circumference?
• In that case, all you have to do is multiply the radius by two, to get the diameter. Then, multiply the diameter by pi to get the circumference.
• how do you turn a fraction into degrees without it being a decimal?
• You can do proportions. You multiply the denominators, then divide the first numerator by that number. Voila! You get the answer, no decimals included.
• At there is a pop up that says" Sal said"obtuse angle" but meant "reflex angle." What is a reflex angle?
• A reflex angle has a measure between 180 and 360 degrees.
• anyone else hear someone talking in the background?
• nvm he said it later on in the video that it was a confrence😅
• At about , couldn't you just subtract pi/2 from 18pi, because 360 degrees minus 350 degrees is 10 degrees? Wouldn't that save a lot of time?
• You could, but I think Sal just did it the long way to demonstrate how its done.