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### Course: Geometry (all content) > Unit 14

Lesson 5: Arc length (from radians)# Challenge problems: Arc length (radians) 1

Solve three challenging problems that ask you to find arc length without directly giving you the arc measure.

## Problem 1

## Problem 2

## Problem 3

## Want to join the conversation?

- I wish there was a little screen we can use to do the work on the computer instead of wasting paper.(29 votes)
- You can use the
**Math Input Panel**app that is already installed on Windows, if your device is running a Windows OS (Vista/7/8/8.1/10). Hope this helps!(36 votes)

- Isn't it just quicker to solve the above problems by finding the arc length first by Arc = radius x angle?

Then subtract this amount from 2 Pi then multiply the result by the radius?

Thanks

Best regards(36 votes)- You cannot find the arc length of CD until you have established a measure for angle CPD or a combined measure of all other angles.(6 votes)

- Is there are way to tell that CD (in question 3) was asking for DC in the clockwise direction, rather than CD in the anti-clockwise direction? I got my answer (correct) only by looking at the answers provided. Is there a convention that says, 'looking for angles is always clockwise'? Or did I miss something such as CD clockwise would be written as CAD,or CBAED, or something?(6 votes)
- When we write an arc with only two letters it implies that it is a minor arc (under 180°) so in this case CD is the arc that goes anti-clockwise from C to D, in other words the small arc.

And yes you are right about the clockwise notation : we typically use a third point to write major arcs (over 180°) in order to avoid any confusion, so CD clockwise could be CAD, CBE and so on.(7 votes)

- I'm so confused about how to find the lengths and the videos just aren't helping me.(6 votes)
- To find the length of an arc, you can apply the formula 2πr * θ / (2π)

-> r * θ.

2πr is the circle circumference.

θ / (2π) is the ratio of the angle of the arc to the whole circle (360 degree = 2π radian)(3 votes)

- why don't the options have 85/2 pi radians? is it a convention not to write radians or something?(4 votes)
- radians is a measure of angles, so if you were measuring a length, you would not use radians, the answer would be in "units" not radians. When you divide the arc measure/radians in a circle, both units are radians, and they cancel, so when you multiply by the circumference in units, then the answer is in units.(6 votes)

- So arc length (in radians) is just
`length = angle*radius`

?(3 votes)- That is correct! For example, the arc length of an angle of 2/5 radians in a circle with a radius of 5 units is 2 units.(2 votes)

- The end part of changing 7pi/36 to 35pi/18,

All i did was multiply 7pi/36 by 10, the radius, then simplify.

Is this an incorrect form/doesn't work in every situation?(2 votes)- This is valid, the arc length will always be the subtended angle multiplied by the radius.(4 votes)

- um can someone explain arc length

central angle

circumference

sorry but sal didnt really hit the subject like i thought(3 votes) - Can somebody explain the logic behind the way of solving all of these questions, I feel like I am just memorizing some formulas , thank you!(3 votes)
- The logic behind behind finding arc lengths and arc measures is related to finding the measure of the central angle that intercepts the arc. As Khan Academy solves them using proportions, it is one way but if you're not finding yourself finding comfortable with proportions as in arc length/circumference = arc measure/360, you can use other ways too.

Most questions ask you to find arc lengths and arc measures, which you can easily find by finding the measure of the central angle the arc subtends and use the 1 or 2 formulas to find arc length.(2 votes)

- How can an angle be measured in radians?(2 votes)
- Radians are just a different way of measuring angles. The definition of what makes a radian a radian is complicated (sal explains why a radian is a radian in the previous lesson), but all you really need to know is that there are 2pi radians in 360 degrees. Using this, you can convert any angle measured in degrees into radians.(3 votes)