Final answer:
The exact value of cos (-π/4) is -1/√2.
Step-by-step explanation:
The exact value of cos (-π/4) is -1/√2.
To find this value, we can use the unit circle. The angle -π/4 is in the third quadrant, where the cosine function is negative. Since the angle -π/4 is a reference angle of π/4, the cosine value will be the same as the cosine of π/4, which is 1/√2.