cos(17pi/8)
cos(17π/8)
But π radians = 180°
17π/8 radians = (17/8) * 180 = 382.5
But this is more than 360°, so we have the basic angle by subtracting 360°
382.5 - 360 = 22.5
cos(17π/8) = Cos22.5
Recall double angle formula for cos.
cos2θ = 2cos²θ - 1
Therefore
cos(2*22.5) = 2cos²(22.5) - 1
cos45 = 2cos²(22.5) - 1
(√2)/2 = 2cos²(22.5) - 1
2cos²(22.5) - 1 = (√2)/2
2cos²(22.5) = (√2)/2 + 1
2cos²(22.5) = (2 + √2) / 2
cos²(22.5) = (2 + √2) / 4
cos22.5 = √( (2 + √2)/4) ................(note)
cos22.5 = √(2 + √2) / 2
Option (a) seems like the closest, comparing to (note) above.
