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### 2.3. Triangles (Planar)

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• Trigonometry involves triangles, and it is necessary to be familiar with the properties of triangles. Triangles are three-sided polygons and contain three angles. The symbol for a triangle is Δ. The three points where the three line segments that make up the triangle intersect or meet are called vertices.

• The sum of the angles in a planar triangle is always 180°. This is found by using the formula for sum of angles in a polygon: (n – 2)180°. Using this formula we find that the sum of the angles of every triangle is 180°. For a triangle, n = 3, (3 – 2)180° = (1)180° = 180°.