## The substended angle of a circle

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!

 PhysOrg.com science news on PhysOrg.com >> Heat-related deaths in Manhattan projected to rise>> Dire outlook despite global warming 'pause': study>> Sea level influenced tropical climate during the last ice age