it equals (−12) because cos(60∘)=12
Explanation:
The reference angle for 240∘ is 60∘ (since 240∘=180∘+60∘)
60∘ is an angle of one of the standard triangles with
cos(60∘)=12
240∘ is in the 3rd quadrant so (either by CAST or noting that the "x-side" of the associate triangle is negative)
cos(240∘)=−cos(60∘)
cos(240∘)=−12
