Texas HS Geometry Question of the Day

Algebraic equation: $m\angle A+m\angle B=m\angle C$ for every triangle.

Coordinate diagram: Triangle with vertices $(0,0)$, $(4,0)$, $(1,3)$; the angles appear to be about 50, 60, and 60 degrees, showing they do not necessarily sum to 180.

Written explanation: Through one vertex, draw a line parallel to the opposite side; the other two interior angles become alternate interior angles with this line, so the three interior angles sit on a straight line and sum to 180.

Numerical example: A triangle with angles 80, 60, and 50 degrees shows the sum as 190.