Points in Quadrant II

A point in quadrant II is the mirror image, in the y-axis, of a point in quadrant I. The mirror keeps the height and flips the across.

Take 120°. Its reference angle is 60°, so the triangle under it is the 60° triangle: across 1/2, height √3/2. Mirrored into quadrant II the across turns negative: the point at 120° is ($\frac{-1}{2}$, $\frac{\sqrt{3}}{2}$), so $\cos\left(120^{\circ}\right) = \frac{-1}{2}$ and $\sin\left(120^{\circ}\right) = \frac{\sqrt{3}}{2}$.