Sine of 240 Degrees/Proof 2

Proof
When $240 \degrees$ is embedded in a Cartesian plane, it makes an angle of $60 \degrees$ with the $x$-axis.

$240 \degrees$ can be found in the third quadrant.

Hence by definition of sine function in the third quadrant, $\sin 240 \degrees$ is negative.

Thus:
 * $\sin 240 \degrees = -\sin 60 \degrees = -\dfrac {\sqrt 3} 2$