Example
	A right circular cone has a semi-vertical angle . Calculate the solid angle at the apex P of the cone.
	Solution :
	The cap on the cone is a part of a sphere of radius R, the slant length of the cone. Using spherical polar coordinates, an area element on the cap is sin , where is the polar angle and is the azimuthal angle. Here, goes from 0 to while goes from 0 to .
	Thus the area of the cap is
	Thus the solid angle at P is

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