Then remember two formulas, "even if it changes strangely, the symbol will look at the quadrant."

"Odd and even changes" refers to:

(1) If k is an even number, then the sign of trigonometric function before α remains unchanged.

(2) If k is an odd number, the sign of the trigonometric function before α should be changed according to the principle of: sin→cos；; cos→sin； Tan → Kurt, Kurt → Tan.

(3) Symbol looking at the quadrant refers to determining the final symbol according to the quadrant where the angle α is located.

Let me give you an example:

sin 1730 = sin( 19×90+20)

Step 1: K = 19 here is an odd number, so change sin to cos；;

Step 2: Make sure that the terminal edge of 1730 is in the fourth quadrant, and then know that the symbol of sin 1730 is "-".

So sin1730 = sin (19× 90+20) =-cos 20.

As for how to judge the symbols of various trigonometric functions in the four quadrants, you can also remember the formula "a full pair; Two sinusoids; The third is cutting; Four cosines ".

The meaning of this 12 formula is:

The four trigonometric functions at any angle in 1 quadrant are all "+";

In the second quadrant, only the sine is "+",and the others are "-";

The tangent function of the third quadrant is "+"and the chord function is "-";

In quadrant 4, only cosine is "+",others are "-".