2、cos(-α)=cosα

3、sin(π/2-α)=cosα

4、cos(π/2-α)=sinα

5、sin(π/2+α)=cosα

Memory skills: odd and even changes, symbols look at quadrants. That is, if the shape is (2k+1) 90 α, the name of the function will become a remainder function, sine will become cosine, cosine will become sine, tangent will become cotangent, and cotangent will become tangent. If the shape is 2k×90 α, the function name remains unchanged.

Other knowledge

The essence is the mapping between any set of angles and a set of ratio variables. The usual trigonometric function is defined in a plane rectangular coordinate system. Its definition field is the whole real number field. The other is defined in a right triangle, but it is incomplete. Modern mathematics describes them as the limit of infinite sequence and the solution of differential equation, and extends their definitions to complex system.

As long as we master the essence and internal laws of trigonometric functions, we will find that there is a strong connection between the formulas of trigonometric functions. And mastering the inherent law and essence of trigonometric function is also the key to learn trigonometric function well.