# The substended angle of a circle

by Kontilera
Tags: angle, circle, substended
 P: 102 Hello! I came across the following formula for the change in the subtended angle (of a circle), when we move our obersvation point $$\vec{x}$$ by $$\delta \vec{x},$$ $$\delta \Omega = \oint \hat{x} \cdot \frac{(\delta \vec{x} \times d\vec{l})}{|\vec{x}|^2}.$$ The integration path is the circumference of the circle. Do you know any litterature or easy derivation that can prove this fact? Thanks for your help, really appreciated!