Trigonometric functions at related angles
Using the geometric symmetry of the unit circle, some trigonometric functions can be established. You can calculate the trigonometric functions of an angle in the second, third or fourth quadrant using its ratio with the first quadrant.
Two angles are aomplementary if they add up to 90 degrees.
If A and B are two angles where A+B=90º, that is, B=90º - A, we have:
sinA = cos B
, so that, sin A = cos(90º-A)
cosA = sin B
, so that, cosA=sin(90º-A)
Similarly, tgA = cotgB