Question

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

Answer 1

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°).