Answer:
The bottom one ....see below
Step-by-step explanation:
Using reference angle of pi/8 and half angle trig identity:
cos ( angle/2) = -sqrt (1 + cos (angle) /2)
cos ( pi/4 / 2) = - sqrt (1+ cos ( pi/4)/2)
cos (pi/8) = - sqrt (1 + sqrt (2) / 2)
= - sqrt (1 + sqrt 2) / sqrt 2 rationalize the denominator
= - sqrt ( sqrt 2 + 2) / 2
re arrange to
- sqrt ( 2 + sqrt2) / 2 the fourth answer
