45. cos \frac{9\pi}{4}

45. cos \frac{9\pi}{4} = cos (\frac{8\pi}{4} + \frac{\pi}{4}) = cos (2\pi + \frac{\pi}{4}) = cos \frac{\pi}{4} = \frac{\sqrt{2}}{2}

Ответ: 2) \frac{\sqrt{2}}{2}