Thread: Another Circle

Circle with radius of 10 in
Chord AB is 14 in
Find measure of minor arc AB and then actual length of minor arc AB

Originally Posted by stones44

Circle with radius of 10 in
Chord AB is 14 in
Find measure of minor arc AB and then actual length of minor arc AB

i'm a bit rusty on my circle theorems, so there may be an easier way to do this--but this is the first idea that came to me. see the diagram below

we can use the law of cosines to find the angle x. once we have that, we can use the length of arc formula to find the desired arc

Hello, stones44!

Here's another way to determine that central angle .

In Jhevon's diagram, draw the apothem to chord AB.

Then: .

Originally Posted by Soroban

Hello, stones44!

Here's another way to determine that central angle .

In Jhevon's diagram, draw the apothem to chord AB.

Then: .

Haha, i didn't see that. that's nice. and a lot simpler than using the cosine rule! As always Soroban does it better

Originally Posted by Jhevon

That's nice. and a lot simpler than using the cosine rule!

While I agree Soroban's is standard and nice, I prefer the way you approached the problem.
Look: .
That is certainly calculator ready; and the actual arclength is 10x.
Don't sell yourself short.

Hello, Jhevon!

As always Soroban does it better.

I certainly am not "better" or "smarter" than you . . . just lazier.
. . I was not born with this gift; it took years of dedicated practice.

so actual arc length?

how is it 10x?

Hello, stones44!

so actual arc length?
how is it 10x?

The arc length formula is: .
. . where is the radius and is the central angle in radians.

Using Jhevon's approach: . radians

Therefore: . inches.

is there another way because I haven't learned that way

Originally Posted by stones44

is there another way because I haven't learned that way

which way are you refering to? Soroban's way or my way? or both?--God forbid

Originally Posted by stones44

is there another way because I haven't learned that way

If you are referring to arc length, no.

-Dan

Originally Posted by stones44

so actual arc length?

how is it 10x?

10 is the radius. and is the angle that subtends the arc. it may have been better for me to call it . so you see the arc length formula is , and that's why it is 10x, since and . there is no other method as far as this part is concerned. if it is everything up to this point that you are unclear on, please be specific as to what the problem is

Do you know the origin of the word radian? How large is one radian?
One radian is the measure of the central angle in any circle that intercepts an arc of the circle equal in length to the radius of the circle. So in any circle, if we have a central angle of 2.5 radians the intercepted arc has length 2.5r. If the circle has radius 10 then any central angle of .35 radians intercepts an arc of length 3.5.

but if the measure of the arc is 88.8, meaning the central angle is 88.8, does that mean the length of the arc is 888? (with radius 10)

Originally Posted by stones44

but if the measure of the arc is 88.8, meaning the central angle is 88.8, does that mean the length of the arc is 888? (with radius 10)

no. you have the angle given in degrees here. you would use a different formula for that. in a circle, there are radians, and no more. anymore than that and it means you are making more than 1 revolution around the circle.

For degrees:

The length of arc is given by:



where is the angle that subtends the arc in degrees