Straight angle

A straight angle is an angle in which the two sides of the angle lie along a single line, pointing in opposite directions. The measure of every straight angle is 180 degrees or $\pi$ radians.

[asy] pair A, B, I;  A = (-1, 0); B = (1, 0); I = (0, 0);  draw(A--B, Arrows); draw(anglemark(B, I, A, 4));  label("$180^\circ$", I, 4N); [/asy]

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