The interior angles of a 9-agon add up to (9 -2)*180° = 1260°. The sum of the given angles is
.. 1139° +x° = 1260°
.. x = 1260 -1139 = 121
The missing angle measure is 121°.
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The supplement of each interior angle is its corresponding exterior angle. The sum of exterior angles of any convex polygon is always 360°. Another way to approach this problem is to add the exterior angles.
