sin(cos^-1(1/-2)

There are two angles that have a cosine of -1/2 between 0° and 360°.

One is 120° and it has a sin value of √3/2 ≈ 0.8660.

The other is 240° and it has a sin value of -√3/2 ≈ -0.8660.

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{sin}}{\left(\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{1}}}{-{\mathtt{2}}}}\right)}\right)} = {\mathtt{0.866\: \!025\: \!403\: \!784}}$$