SOLUTION:

Question 567171:
Answer by nottohave(4) (Show Source): You can put this solution on YOUR website!
Draw a circle on x,y origin. Top right: I - which is ALL possi. value for trigs. Top left: II - possible trig value only for Sine. Bottom left: III - possible trig values only for Tan . The left over is Cosine.
Cos x = negi.1/2 mean the answer is on II or III ( also II and III are the x negative values ).
Cos x = 1/2 and x= 1Pie/3 = 60 degree ( on the I ) . But 2 pie/3 (on the II) and 4 pie/3 (on the III) given negative x-value so it will be -1/2.