Suppose we pick a random point `B` on the terminal side, and draw "straight down" to point `C` of the initial side, we get a right triangle `∆ABC`. Now Let us pick another random point `D` on the terminal side, and draw "straight down" to point `E` of the initial side, we get a right triangle `∆ADE`. `∆ABC` and `∆ADE` are similar. Property of similar triangle: `(AB)/(AD) = (BC)/(DE) = (AC)/(AE)` From this property we can get: `(DE)/(AD) = (BC)/(AB)` In other words, the ratio of the opposite side of `angle A` to the hypotenuse is a constant. Mathematicians named this ratio sine(`sin`)