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It's not quite clear what you want to do but if you want to:
1. Create a single straight line that has the same length as the arc length of the arc then use the following formula:
L = R * theta
where R is the radius of the arc and theta is the angle, in radians, subtended by the arc. Draw a line of length L and you are all set.
2. If you want to represent an arc by a series of straight line segments where the sum of the lengths of the segments equals the arc length then you may need to do a little math. For example, let's say you wanted to represent a circle with a polyline of 8 segments such that the length of the polyline is the same as the circle. Let's assume that the diameter of the circle is 1.0. Therefore its circumference is pi, 3.14159265...
If we draw a circle (red below) of diameter 1.0 ( r = 0.5) and the use the polygon command to create an 8 sided polygon inscribed in the circle we would get a polyline (cyan) with a length less than the circumference of the circle and the circumscribed circle (yellow) would be too long. To find the radius of a circle where we could construct an 8 sided polygon of length pi we can determine the length of each side of the octagon
S = pi/8 = 0.392699
we can write that
sin(360/16) = S/2 / r
or r = 0.392699/2/sin(22.5°)
r = 0.51208597
The green circle below is this circle and the white line the 8 sided polyline of length 3.14159265.
A similar approach could be taken with an arc although if the number of segments required for the polyline may not enable an exact solution.