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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. Supplementary angles.
If A and B are two angles where A+B=180º , that is, B=180º  A, we have:
sinA = sin B, so that, sin A = sin(180ºA) cosA = cos B , so that, cosA = cos(180ºA) Similarly tanA = tanB Using these formulas, you can calculate the trigonometric functions of an angle in the second quadrant if you know the trigonometric functions of its supplementary angle. 