One example of calculating the sum of a set of angles, even without knowing their individual measures, is that the three internal angles of a triangle always add to . But what about other polygons or more complex figures?
What is the sum of the green angles in the image below?
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There are three triangles in this diagram, so the sum of all their interior angles is 1 8 0 ∘ + 1 8 0 ∘ + 1 8 0 ∘ = 5 4 0 ∘ . The three unshaded angles at the bottom form a straight angle with a measure of 1 8 0 ∘ . The total measure of the green angles is the measure of all interior angles minus the measure of the straight angle: 5 4 0 ∘ − 1 8 0 ∘ = 3 6 0 ∘ .