Write the following expression as a function of a positive acute angle. Use "+" to indicate a positive value and "-" to indicate a negative value.

cos (-240°) = cos

cos (-240°) = cos 120°; the "adjacent sides" and the "hypotenuses" are the same in both cases. The supplement of 120° is 60°, and 60° is a positive acute angle. Since cos (-240°) is negative, we need to write

cos (-240°) = - cos (60°).

Check: cos (60°) = 1/2, so - cos (60°) = - 1/2, which is the value of cos (-240°).