Question 79068
To find the angle, lets take the inverse cosine of {{{cos(120)}}} to get

{{{cos(cos(120))^-1=pi/3}}}

So the angle is {{{pi/3}}}


Another way to do this is to notice that the given angle is 120 degrees (which is {{{2pi/3}}} radians) So to find the reference angle, just subtract this angle from  {{{pi}}} radians


{{{pi-2pi/3=3pi/3-2pi/3=(3pi-2pi)/3=pi/3}}}