# Find exact value of \cos (\frac{9\pi}{4}) How do you get (\frac

Find exact value of $\mathrm{cos}\left(\frac{9\pi }{4}\right)$
How do you get $\left(\frac{\pi }{4}+2\pi \right)$
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wheezym
$\frac{9\pi }{4}=\frac{\pi }{4}+\frac{8\pi }{4}=\frac{\pi }{4}+2\pi$
so $\frac{\pi }{4}$ is the reference angle $\mathrm{\angle }{45}^{o}$
$\mathrm{cos}\left(\frac{\pi }{4}\right)=\frac{\sqrt{2}}{2}$