زاویه های معروف و مهم
The radian (SI symbol rad) is the SI unit for measuring angles, and is the standard unit of angular measure used in many areas of mathematics. The length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it subtends; one radian is just under 57.3 degrees(expansion at A072097). The unit was formerly an SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an SI derived unit.
رادیان زاویه مرکزی مقابل به کمانی از دایره است که طول آن با شعاع دایره برابر است. یعنی زاویه مرکزیِ متناظر با محیط دایره، مساویِ رادیان و اندازه زاویه نیم صفحه، رادیان و اندازه زاویه قائمه، رادیان است.
زاویه در درجه = زاویه در رادیان .
به عنوان مثال:
و بلعکس: با ضرب در درجه، رادیان بدست میآید:
جدول زیر تبدیل چند زاویه پرکاربرد را نمایش میدهد:
Radian describes the plane angle subtended by a circular arc as the length of the arc divided by the radius of the arc. One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle. More generally, the magnitude in radians of such a subtended angle is equal to the ratio of the arc length to the radius of the circle; that is, θ = s / r, where θ is the subtended angle in radians, s is arc length, and r is radius. Conversely, the length of the enclosed arc is equal to the radius multiplied by the magnitude of the angle in radians; that is, s = rθ.
Conversion between radians and degrees
As stated, one radian is equal to 180/π degrees. Thus, to convert from radians to degrees, multiply by 180/π.
Conversely, to convert from degrees to radians, multiply by π/180.
Radians can be converted to turns (complete revolutions) by dividing the number of radians by 2π.
Radian to degree conversion derivation
The length of circumference of a circle is given by , where is the radius of the circle.
So the following equivalent relation is true:
[Since a sweep is needed to draw a full circle]
By the definition of radian, a full circle represents:
Combining both the above relations:
Advantages of measuring in radians
In calculus and most other branches of mathematics beyond practical geometry, angles are universally measured in radians. This is because radians have a mathematical “naturalness” that leads to a more elegant formulation of a number of important results.
Most notably, results in analysis involving trigonometric functions are simple and elegant when the functions’ arguments are expressed in radians. For example, the use of radians leads to the simple limit formula
which is the basis of many other identities in mathematics, including
Because of these and other properties, the trigonometric functions appear in solutions to mathematical problems that are not obviously related to the functions’ geometrical meanings (for example, the solutions to the differential equation , the evaluation of the integral , and so on). In all such cases it is found that the arguments to the functions are most naturally written in the form that corresponds, in geometrical contexts, to the radian measurement of angles.
The trigonometric functions also have simple and elegant series expansions when radians are used; for example, the following Taylor series for sin x :
If x were expressed in degrees then the series would contain messy factors involving powers of π/180: if x is the number of degrees, the number of radians is y = πx / 180, so
Mathematically important relationships between the sine and cosine functions and the exponential function (see, for example, Euler’s formula) are, again, elegant when the functions’ arguments are in radians and messy otherwise.
Although the radian is a unit of measure, it is a dimensionless quantity. This can be seen from the definition given earlier: the angle subtended at the centre of a circle, measured in radians, is equal to the ratio of the length of the enclosed arc to the length of the circle’s radius. Since the units of measurement cancel, this ratio is dimensionless.