Sectors
http://www.regentsprep.org/Regents/math/geometry/GP16/CircleSectors.htm
http://www.brightstorm.com/math/geometry/area/area-of-a-sector
Segments
http://www.brightstorm.com/math/geometry/area/area-of-a-segment
Anulus
http://www.brightstorm.com/math/geometry/area/area-of-an-annulus
