Radian. We will consider a circle of radius , and call it the unit circle. Then an arc length of the unit circle is called a radian, typically denoted by . Recall that the circumference of the unit circle is . For example, , and are equal to , and .
Special right triangles. The right triangle with adjacent side and opposite side gives the hypotenuse and the angle .
The right triangle with opposite side and hypotenuse gives the adjacent side and the angle .