Let us recall the formula for the circumference of a circle. That one is 2pi r. r is the radius of the circle and 2pi is the angle in radian measure subtended by the entire circle at the centre. If this is so, then any arc length 'l' will be equal to the product of the angle in radian measure subtended by the arc at the centre and the radius.
So l = theta r. Say theta is the angle subtended by the arc at the centre.
Therefrom, r = l / Theta.
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you need to know the formula the arc length is equal to the radius times the angle made by the length of arc s = r(theta) s=arc length r=radius theta=angle
Length = angle˚/360˚ x 2∏r
The length of the arc is r*theta where r is the radius and theta the angle subtended by the arc at the centre of the circle. If you do not know theta (or cannot derive it), you cannot find the length of the arc.
The radius is a cord, is it not?
You can use the cosine rule and the three lengths of the triangle to find the central angle X, (in radians). Then the length of the arc is r*X units.