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a)
\sin\ \frac{\pi}{6}=\frac{1}{2}
b)
\cos\ \frac{\pi}{6}=\frac{\sqrt{3}}{2}
c)
\sin\ \frac{3\pi}{4}=\frac{1}{\sqrt{2}}
d)
\cos\ \frac{3\pi}{4}=-\frac{1}{\sqrt{2}}