A straight angle is 180 degrees. Two angles on a straight line add to 180. Complementary and supplementary as a set are in complementary and supplementary angles.
One line, two pieces
If one piece is 65°, the other is 115°. A full circle is 360°, in degrees in a circle. A triangle still adds to 180, in triangle angle sum.
The short list is in ACT Math formulas.
The bottom line
Straight line: 180°. Subtract the given piece from 180.
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