Question

What is the exact value of cos (7pi/8)

Answer 1

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