Proof: By Euclid
(related to Corollary: Triangulation of an Ngon and Sum of Interior Angles)
 By Prop 1.32, the sum of all angles in a plane triangle is $180^\circ.$
 If a convex nsided figure is triangulated, we will obtain \((n2)\) triangles.
 Therefore, the sum of its angles equals \(2(n2)\) right angles.
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References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclidâ€™s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"