Tan's two-angle sum formula

There is no need to push this formula, just remember that the trigonometric functions of the four quadrants are positive and negative.

Remember two words, parity unchanged, symbols look at quadrants, such as:

For tan (pie /2 *k+a)=?

Look at k first, whether it is odd or even. Odd numbers will become cot, and even numbers will remain unchanged. Then take A as the angle of the first quadrant, rotate (Pi /2 *k+a) to see which quadrant it is, and then see if tan is just negative in that quadrant. Just add this symbol in front of the canvas bed at the back.

Ru Tan (Pai /2 +a)=? First of all, it is strange that k= 1 should be changed to cot, where a is the angle of the first quadrant and Pi /2 +a is the angle of the second quadrant, which is negative, so it is-cot a.