the angle on a straight line is 180 degrees, the angle of semicircle at centre is 180 degrees
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The interior angle and the exterior angle are supplementary. That's why you learned about supplementary angles ... or at least they were presented in class ... before polygons.
A supplementary angle can have any value - depending on the first angle.
Supplementary angles add up to 180 degrees
Supplementary angles total 180 degrees. If you have one angle, subtract it from 180 to find its supplement.
A triangle is a polygonal shape, not an angle and therefore, cannot have a supplementary angle.