Trigonometric function is a transcendental function in elementary functions in mathematics. Their 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.

Deduction method:

The trigonometric function of odd multiples of 90+α and the absolute value of α trigonometric function are complementary. The trigonometric function of even multiples of 90+α is the same as the absolute value of α. That is, "odd parity is the same, odd parity is the same."

Take α as an acute angle (note that it is "regarded"), and take the sign of trigonometric function according to the quadrant of the obtained angle. That is, "Like a finite number, the symbol looks at the quadrant" (or "Even if it changes singularly, the symbol looks at the quadrant").