Shall I teach you how to memorize inductive formulas? If you can learn, you don't need to remember so many formulas in the textbook, because it is obtained by integrating the formulas classified in the textbook.
Converting the angle alpha into the form of kπ/2+θ or kx90+θ,
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 "-".
If you can understand the meaning of this passage, there is actually only one inductive formula. In teaching, I never ask students to remember the inductive formula in the textbook, but ask them to understand the inductive formula according to the above passage. The effect is very good. Please try it.