### Instruction: Compute $\cos 240^\circ$.### Response: 240 degrees can be expressed as $180^\circ+60^\circ$
So we can express cos 240 as cos (180+60)
Or as cos 180 cos 60 - sin 180 sin 60
But cos 180 = -1 and sin 180 = 0, so that expression becomes -1*cos 60 - 0*sin 60
Which is just -cos 60.
Right, so cos 240 = -cos 60.