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My doubt is how Arc(ab) = L $\gamma$. Arc (ab) = R$\theta$ makes sense as that's the arc of that circle.

  • $\begingroup$ Because $\gamma$ is a small angle. $\endgroup$ – lucas Jun 3 '16 at 10:48

If the angle $\gamma$ is small we can approximate the arc $AB$ as a straight line:

length of AB

Then the length $AB$ is simply:

$$ AB = L\tan\gamma $$

Since $\gamma$ is small $\tan\gamma\approx\gamma$ and the equation becomes:

$$ AB \approx L\gamma $$


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