`c)`
`text{Ta có}`
`cos ((pi)/4) = 2cos^2 ((pi)/8) - 1`
`-> (\sqrt{2})/2 + 1 = 2cos^2 ((pi)/8)`
`-> cos ((pi)/8) = sqrt{(\sqrt{2}(\sqrt{2} + 1))/4}`
`-> cos ((pi)/8) = (\sqrt{\sqrt{2} + 2})/2`
`d)`
`text{Ta có}`
`(sin x)/(1 + cos x)`
`=` $\dfrac{sin (2.\dfrac{x}{2})}{1 + cos (2.\dfrac{x}{2})}$
`=` $\dfrac{2sin (\dfrac{x}{2}).cos (\dfrac{x}{2})}{1 + 2cos^2 (\dfrac{x}{2}) - 1}$
`=` $\dfrac{sin (\dfrac{x}{2})}{cos (\dfrac{x}{2})}$
`= tan (x/2)`
