The double angle formula uses the trigonometric function of this angle to represent the trigonometric function of the double angle. It can be used to simplify the calculation formula and reduce the number of trigonometric functions in calculation, and it is also widely used in engineering. Double angle formula is a very practical formula in trigonometric function. For example:

Half-angle formula is a formula for finding trigonometric functions such as sine, cosine and tangent of an angle (such as A). For example:

The trigonometric function difference angle formula, also known as trigonometric function subtraction theorem, is the relationship between the trigonometric function of the sum (difference) of several angles and the trigonometric function of each angle. For example:

Double angle formula, half angle formula and difference angle formula (sum difference formula) are the basic formulas of trigonometric function.