Answer:
150° and 210°
Step-by-step explanation:
1) according to the condition the required cosine is <0, it means the required angles are in the II-d and III-d quaters (see the attached picture no. 1);
2) it is known, that arccos(√3/2)=30°- the angle is between 0° and 90°, - but the requred angles are between 90° and 270° (see the picture no. 2), then
3) (angle)₁=180°-arccos(√3/2) and (angle)₂=180°+arccos(√3/2), then finally
(angle)₁=180°-30°=150°; (angle)₂=180°+30°=210°.
PS. note, the suggested way is not the only one.
