**The sine of pi is zero.** In trigonometry, pi is known as the root or inflection point of the sine function. When expressed in radians, the sine of pi is equivalent to the sine of a 180-degree or 360-degree angle, both of which are also equal to zero.

The sine of a segment of a circle with pi radians can be determined by forming a right triangle and determining the length of each side. Using the Pythagorean theorem, the sine function evaluates the ratio of the side opposite the angle over the hypotenuse of the right triangle. The sine function is related to the trigonometric functions cosine and tangent.