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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. Angles that differ by 180º
If A and B are two angles that B  A= 180º, that is,
B=180º + A, then: sinA = sin B, so that, sin A = sin(180º+A) cosA = cos B , so that, cosA = cos(180º+A) Similarly tanA = tanB Using these formulas, you can calculate the trigonometric functions of an angle in the third quadrant if you know the trigonometric functions of the angle that differs with it 180º.
