Answer:
9. 1620°
10. 360°
11. 180°
Step-by-step explanation:
The answers of problems 10 and 11 give you the way to find the answer to problem 9.
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10. The sum of exterior angles of any convex polygon is 360°.
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11. An internal angle and an external angle are a linear pair, so always sum to 180°.
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9. At every vertex, the sum of interior and exterior angles is 180°, so the total of those angles at all vertices is n·180°. Of that total, 360° is the sum of exterior angles, so the total of interior angles of an n-gon is ...
n·180° -360° = (n -2)·180°
For an 11-gon, the sum of interior angles is ...
(11 -2)·180° = 1620°
