Answer:
∠CDE = 170°
Explanation:
You want the measure of angle D in the heptagon shown.
Angles
The sum of angles in a heptagon is 180°(7 -2) = 900°. This means ...
(2x -13) +(2x +9) +(2x +11) +(3x -1) +(2x -7) +(2x -3) +(3x -8) = 900
16x = 912 . . . . . . . add 12
x = 912/16 = 57
Angle D = 3x -1 = 3(57) -1 = 170
The measure of angle CDE is 170°.
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Additional comment
The sum of interior angles of an n-sided polygon is (n -2)·180°.
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