In any triangle, if one of the sides is extended, the exterior angle is greater than both the interior and opposite angles.
Exterior Angle Theorem
See also
Exterior AngleExplore with Wolfram|Alpha
References
Dunham, W. Journey through Genius: The Great Theorems of Mathematics. New York: Wiley, p. 41, 1990.Referenced on Wolfram|Alpha
Exterior Angle TheoremCite this as:
Weisstein, Eric W. "Exterior Angle Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ExteriorAngleTheorem.html