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. Opposite angles. If A and B are two angles that B - A= k·360º where k is an integer, then:
sinA = -sin B, so that, sin A = -sin(-A) cosA = cos B , so that, cosA = cos(-A) Similarly tanA = - tanB Using these formulas, you can calculate the trigonometric functions of an angle in the fourth quadrant if you know the trigonometric functions of its opposite angle. |