INTRODUCTION TO TRIGONOMETRY
作者：SREEKARA NARAYANA SREEKARA NARAYANA
1. 1. In a right triangle ABC, right-angled at B, sin A = side opposite to angle A side adjacent to angle A , cos A = hypotenuse hypotenuse tan A = side opposite toangle A side adjacent to angle A .
2. 2. 1 1 1 sin A , cosec A = ; sec A = ; tan A = tan A = sin A cos A cot A cos A
3. 3. If one of the trigonometric ratios of an acute angle is known, the remaining trigonometric ratios of the angle can be easily determined.
4. 4. The values of trigonometric ratios for angles 0°, 30°, 45°, 60° and 90°.
5. 7. sin^2 A + cos^2 A = 1, sec^2 A – tan^2 A = 1 for 0° ≤ A < 90°, cosec^2 A = 1 + cot^2 A for 0° < A ≤ 90º.
6. 5. The value of sin A or cos A never exceeds 1, whereas the value of sec A or cosec A is always greater than or equal to 1.
7. 6. sin (90° – A) = cos A, cos (90° – A) = sin A; tan (90° – A) = cot A, cot (90° – A) = tan A; sec (90° – A) = cosec A, cosec (90° – A) = sec A.
