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  2. Exterior angle theorem - Wikipedia

    en.wikipedia.org/wiki/Exterior_angle_theorem

    Consider a spherical triangle one of whose vertices is the North Pole and the other two lie on the equator. The sides of the triangle emanating from the North Pole (great circles of the sphere) both meet the equator at right angles, so this triangle has an exterior angle that is equal to a remote interior angle. The other interior angle (at the ...

  3. Sum of angles of a triangle - Wikipedia

    en.wikipedia.org/wiki/Sum_of_angles_of_a_triangle

    Triangle postulate: The sum of the angles of a triangle is two right angles. Playfair's axiom: Given a straight line and a point not on the line, exactly one straight line may be drawn through the point parallel to the given line. Proclus' axiom: If a line intersects one of two parallel lines, it must intersect the other also. [3]

  4. Solution of triangles - Wikipedia

    en.wikipedia.org/wiki/Solution_of_triangles

    If b ≥ c, then β ≥ γ (the larger side corresponds to a larger angle). Since no triangle can have two obtuse angles, γ is an acute angle and the solution γ = arcsin D is unique. If b < c, the angle γ may be acute: γ = arcsin D or obtuse: γ ′ = 180° − γ.

  5. Triangle - Wikipedia

    en.wikipedia.org/wiki/Triangle

    The included angle for any two sides of a polygon is the internal angle between those two sides.) If and only if three pairs of corresponding sides of two triangles are all in the same proportion, then the triangles are similar. [b] Two triangles that are congruent have exactly the same size and shape. All pairs of congruent triangles are also ...

  6. Euclidean geometry - Wikipedia

    en.wikipedia.org/wiki/Euclidean_geometry

    The sum of the angles of a triangle is equal to a straight angle (180 degrees). [14] This causes an equilateral triangle to have three interior angles of 60 degrees. Also, it causes every triangle to have at least two acute angles and up to one obtuse or right angle.

  7. Isosceles triangle - Wikipedia

    en.wikipedia.org/wiki/Isosceles_triangle

    In an isosceles triangle that has exactly two equal sides, the equal sides are called legs and the third side is called the base. The angle included by the legs is called the vertex angle and the angles that have the base as one of their sides are called the base angles. [6] The vertex opposite the base is called the apex. [7] In the ...

  8. Parallel postulate - Wikipedia

    en.wikipedia.org/wiki/Parallel_postulate

    The sum of the angles in every triangle is 180° (triangle postulate). There exists a triangle whose angles add up to 180°. The sum of the angles is the same for every triangle. There exists a pair of similar, but not congruent, triangles. Every triangle can be circumscribed.

  9. Pythagorean theorem - Wikipedia

    en.wikipedia.org/wiki/Pythagorean_theorem

    In mathematics, the Pythagorean theorem or Pythagoras' theorem is a fundamental relation in Euclidean geometry between the three sides of a right triangle.It states that the area of the square whose side is the hypotenuse (the side opposite the right angle) is equal to the sum of the areas of the squares on the other two sides.