Definition:Sine/Definition from Circle/First Quadrant

Definition

 * TrigRatiosFirstQuadrant.png

Consider a unit circle whose center is at the origin of a cartesian coordinate plane.

Let $P = \left({x, y}\right)$ be a point in the first quadrant.

Let $\theta$ be the angle made by $OP$ with the $x$-axis.

Then the sine of $\theta$ is defined as being the measurement $y$.