#1**+20 **

You would use arccosine to solve this problem, since you have the cosine and need the angle:

cos(x) = -1/2

x = acos(-1/2)

x = $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)} = {\mathtt{120^{\circ}}}$$

* Which is also equal to 2pi/3 radians.

kitty<3 May 19, 2015

#1**+20 **

Best Answer

You would use arccosine to solve this problem, since you have the cosine and need the angle:

cos(x) = -1/2

x = acos(-1/2)

x = $$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{cos}}^{\!\!\mathtt{-1}}{\left({\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right)} = {\mathtt{120^{\circ}}}$$

* Which is also equal to 2pi/3 radians.

kitty<3 May 19, 2015