How to calculate arc length in unit circle

[subuntu]

http://www.up98.org/upload/server1/01/z/cllb59cvnwaigmmar6b5.jpeg

What is the method of calculating arc length in In the image above .
x & y is known
Thanks .

 

[subuntu]

Feuilleton :
Obviously, the use of calculators and trigonometric tables is not allowed

 

[Ivan92]

There is a formula for it.

s = r θ, where θ is in radians.

You shouldn't need a Trig table, you should know the table. The angles in the unit circle are special angles which you should know by heart.

 

[eumyang]

There is a formula for it.

s = r θ, where θ is in radians.

It looks like the OP already knows this, because outside of the circle he/she writes: "arc length = θ = ?" (since r = 1).

As for knowing the table, that is helpful ONLY if x, y, and r form one of the two special triangles. If x = √3/2 and y = 1/2, then sure, we can find the arc length no trouble. But what if x = 0.6 and y = 0.8? We would need to make use of the inverse sine, wouldn't we?