Sin? α=( 1-cos2α)/2 .

Tan? α=( 1-cos 2α)/( 1+cos 2α).

Deduction process of power reduction formula;

The angle-doubling formula is used to improve the power, and the formula cos2α can be modified to obtain the formula for reducing the power:

cos2α=cos？ α-sin？ α=2cos？ α- 1= 1-2sin？ α。

∴cos？ α=( 1+cos2α)/2 .

Sin? α=( 1-cos2α)/2 .

The formula of decreasing power is the formula of decreasing exponential power from quadratic to 1, which can reduce the trouble of quadratic.

Introduction of trigonometric function;

Trigonometric function is one of the basic elementary functions, which takes the angle (the most commonly used radian system in mathematics, the same below) as the independent variable, and the angle corresponds to the coordinates where the terminal edge of any angle intersects with the unit circle or its ratio as the dependent variable. It can also be equivalently defined as the lengths of various line segments related to the unit circle.

Trigonometric function plays an important role in studying the properties of geometric shapes such as triangles and circles, and is also a basic mathematical tool for studying periodic phenomena. In mathematical analysis, trigonometric function is also defined as the solution of infinite series or specific differential equation, which allows its value to be extended to any real value or even complex value.