Right angle
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In geometry and trigonometry, a right angle is an angle of 90 degrees, corresponding to a quarter turn (that is, a quarter of a full circle). It can be defined as the angle such that twice that angle amounts to a half turn, or 180°.[1]
Lines that are at a right angle to each other are perpendicular, an important geometrical property. The presence of a right angle in a triangle is the defining factor for right triangles.
A right angle may be expressed in different units:
- 90°
- π/2 radians
- 100 grad (also called grade, gradian, or gon)
- 8 points (of a 32-point compass rose)
- 6 hours (astronomical hour angle)
- ∞% grade on the tangent scale
- 100% grade on the sine scale.
In mathematics, right angle can be an additional unit of angle[2]. 1r=90°=π/2 rad=100g
The term is a calque of Latin angulus rectus; here rectus means "upright", referring to the vertical perpendicular to a horizontal base line. Throughout history carpenters and masons have known a quick way to confirm if an angle is a true "right angle." It is called the "Rule of 3-4-5." From the angle in question, running a straight line along one side exactly three units in length, and along the second side exactly four units in length, will create a hypotenuse (the longer line opposite the right angle which connects the two measured endpoints) of exactly 5 units in length. This measurement can be made quickly and without technical instruments. The geometric law behind the measurement is the Pythagorean theorem ("The square of the hypotenuse of a right triangle is equal to the sum of the squares of the two adjacent sides").
[edit] See also
[edit] References
- ^ Lindahl, G. (1987). Euklides Geometri. Stockholm, Natur och kultur: ISBN 91-27-72185-X
- ^ * convertworld.com angles
a right angle is exactly 90 degrees
