Where do ①sin, cos and tan appear? -In a right triangle.

How to use sin, cos and tan? In a right triangle, according to the definition, formula or equation? Do the math.

③ What if there is no right angle? -As an auxiliary line, construct a right triangle. (This is a common situation)

④ What if I don't know the trigonometric function value of an angle? -Find the trigonometric function value of equal angle.

What if you only know one of ⑤sin, cos and tan? -Set an unknown number, indicate the length of three sides, and then find the other two trigonometric functions. (For example: sinx=3/5, let the opposite side be 3x, the hypotenuse be 5x, and the other right angle side be 4x by Pythagorean theorem).