∠, the angle symbol
In geometry, an angle is the figure formed by two rays sharing
a common endpoint, called the vertex of the
angle. The magnitude of the angle is the "amount of rotation"
that separates the two rays, and can be measured by considering the length of
circular arc swept out when one ray is rotated about the vertex to coincide
with the other (see "Measuring angles", below). Where there is no
possibility of confusion, the term "angle" is used interchangeably
for both the geometric configuration itself and for its angular magnitude
(which is simply a numerical quantity).
Acute (a), obtuse (b), and straight (c) angles. Here, a and b are supplementary
angles.
Angles equal to 1/2 turn (180° or two right angles) are called straight
angles. (Our
solved example in mathguru.com uses this concept)
Two
angles that sum to a straight angle (180°) are called supplementary
angles.
The difference between an angle and a
straight angle (180°) is termed the supplement of the angle.
An
angle equal to 1/4 turn (90° or π/2
radians) is called a right angle.
Two lines that form a right angle are
said to be perpendicular or orthogonal. (Our
solved example in mathguru.com uses this concept)
http://en.wikipedia.org/wiki/Angle
The
above explanation is copied from Wikipedia, the free encyclopedia and is
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Unported License.