An exterior angle of a triangle equals the sum of the two remote interior angles. If those interiors are 50 and 70, the exterior is 120. Interior sum is still 180, in triangle angle sum.
Remote means not adjacent
Extend one side. The angle outside, next to an interior angle, is the exterior. It does not equal its adjacent interior; those two are supplementary. Triangle inequality is about sides, in the triangle inequality.
Complementary and supplementary pairs are in complementary and supplementary angles. The short list is in ACT Math formulas.
The bottom line
Exterior = sum of the two interiors that do not touch it.
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