Trigonometry is a branch of mathematics that studies relationships between side lengths and angles of triangles. It's primarily concerned with the six trigonometric functions: sine (sin), cosine (cos), tangent (tan), cosecant (csc), secant (sec), and cotangent (cot). These functions relate an angle of a right-angled triangle to the ratios of its side lengths. For example, in a right-angled triangle, the sine of an angle is the ratio of the length of the side opposite the angle to the length of the hypotenuse. That's 2 down. 3 left today — send the next one.