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An equilateral triangle is a triangle in which all three sides have the same length, and all three angles are equal. Because of these properties, the equilateral triangle is a regular polygon, occasionally known as the regular triangle. It is the special case of an isosceles triangle by modern definition, creating more special properties.
This is equivalent to saying that one triangle (or its mirror image) is an enlargement of the other. Any two pairs of sides are proportional, and the angles included between these sides are congruent: [6]
Thus, the first two triangles are in the same equivalence class, while the third and fourth triangles are each in their own equivalence class. In mathematics , when the elements of some set S {\displaystyle S} have a notion of equivalence (formalized as an equivalence relation ), then one may naturally split the set S {\displaystyle S} into ...
In geometry, an isosceles triangle (/ aɪ ˈ s ɒ s ə l iː z /) is a triangle that has two sides of equal length or two angles of equal measure. Sometimes it is specified as having exactly two sides of equal length, and sometimes as having at least two sides of equal length, the latter version thus including the equilateral triangle as a special case.
The reverse triangle inequality is an equivalent alternative formulation of the triangle inequality that gives lower bounds instead of upper bounds. For plane geometry, the statement is: [19] Any side of a triangle is greater than or equal to the difference between the other two sides. In the case of a normed vector space, the statement is:
These equivalent statements include: There is at most one line that can be drawn parallel to another given one through an external point. (Playfair's axiom) 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.
AAS (angle-angle-side): If two pairs of angles of two triangles are equal in measurement, and a pair of corresponding non-included sides are equal in length, then the triangles are congruent. AAS is equivalent to an ASA condition, by the fact that if any two angles are given, so is the third angle, since their sum should be 180°.
Similar right triangles illustrating the tangent and secant trigonometric functions Trigonometric functions and their reciprocals on the unit circle. The Pythagorean theorem applied to the blue triangle shows the identity 1 + cot 2 θ = csc 2 θ, and applied to the red triangle shows that 1 + tan 2 θ = sec 2 θ.