Question

cos(π/8)

Answer 1

You could use half angle formula.

Answer 2

\[
\cos(\pi/4)
\]Is trivial.

Answer 3

\[\cos \frac{x}{2}=\pm \sqrt{\frac{1+\cos x}{2}}\]

Answer 4

Since it is in the 1st quadrant it is positive.

Answer 5

but here we will take + sign as pi/8 lies in first quadrant.