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Well, drawing out the diagram; you should have one triangle with all 3 sides with values. Lets call the radius 'r' , the chord length 'c' and the arc length 'a'. Your triangle should have two sides labelled each as 'r', and 'c'should be the last side.

Firstly, we know that the rule for the arc length is radius x theta (radians).

So we need a theta. To find theta, we can use the cosine rule, since we have all 3 sides and no angle.

The cosine rule is a^2=b^2+c^2-2bcCosA. We can transpose this into: CosA=(b^2+c^2-a^2)/2bc.

If we sub in our 'c' as the 'a' in the rule above and the two 'r's as 'b' and 'c', we get our new rule as CosC=(r^2+r^2-c^2)/2rr. We can then simplify that to be CosC = (2r^2-c^2)/2r^2. After that, inverse cosine the value and you should get angle C.

Finally sub in your values into the arc length rule: radius x theta, and you should get your answer.

P.S. Make sure your calculator is in radians.

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Q: How do you find length of arc knowing only the chord of arc and radius?
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