sin 240 = sin( 180+60)=-sin 60；

sin240=sin(270-30)=-cos30 .

The above 180 degree is an even number (2) times of 90 degrees, and the result is still the original function (sine).

And 270 degrees is an odd (3) times of 90 degrees, and the result becomes the cosine of the original function.

Because the original angle of 240 degrees is the angle of the third term, the sign of the original function is negative.

"Parity" means that the degree in front of the angle is a multiple of 90 degrees. If it is an even number, the function name remains unchanged; If it is an odd number, it becomes its complementary function (sine and cosine transform, sine and cotangent transform, sine and cotangent transform).

"Symbol looking at quadrant" means that the symbol of the original function in the quadrant where the original angle is located should be obeyed.

Odd and even refer to odd multiples or even multiples of half π, and changing or unchanging means that the name of trigonometric function remains unchanged. If it is an odd multiple of half π, the name of trigonometric function changes, sin changes to cos, and cos changes to sin. If it is an even multiple, it remains unchanged. Symbols refer to signs. For example, sin(x+π/2)=cosx, because it is followed by half π 1 times, which is an odd multiple, so sin should become cos. Suppose x is [0,90], then X+π/2 is [sin 180], then sin90.