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In geometry, a radian is a unit used to measure angles. The radian comes from the length of the circle’s radius. The segment of a circle’s circumference that corresponds to the angle made by two radius lines makes an arc. The angle that this arc creates, when you draw lines from its starting and end points to the circle’s center, is one radian. Though the radian might appear odd and complicated at first, it simplifies equations in math and physics.

#### TL;DR (Too Long; Didn't Read)

In geometry, a radian is a unit based on the circle and used to measure angles. It eases calculations in advanced types of math.

Outside of physics and advanced math, degrees are typically more familiar units for angular measurements. A circle, for example, has 360 degrees, a triangle has 180 and a right angle has 90. By contrast, a full circle has 2 × π (pi) radians, a triangle has π radians and a right angle is π/2 radians. A circle has a whole number of degrees, whereas in radians the value is an irrational number, so radians at first blush might seem strange. On the other hand, you might express fractions of a degree as a decimal, or as the minutes, seconds and decimal seconds you also use with time, so the degree has issues of its own.

## Easier and Harder

Degree measurements are typically easier to deal with than radians for basic arithmetic and trigonometry; you seldom have to deal with fractions of π when expressing an angle. But for calculus and other advanced math, it turns out that radians are easier. For example, the power series for the sine function in radians is as follows:

\sin(x) = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \frac{x^9}{9!} + ...

In degrees, the function looks like this:

\sin(x) = (π × x/180) - \frac{(π × x/180)^3}{3!} + \frac{(π × x/180)^5}{ 5!} - \frac{ (π × x/180)^7}{7!} + \frac{(π × x/180)^9}{9!} + ...

For this power series, note that you need to repeat the “π × ​x​/180” for every term – a lot of extra writing and calculation compared to the neater, more compact equivalent in radians. The radian comes from the natural geometry of a circle rather than a division by an arbitrary number, as degrees do. Because radians make many calculations easier, mathematicians think of the unit as more “natural” than degrees.