$\\$ $\cos(\frac{5\pi}{12})=\cos(\frac{2\pi+3\pi}{12})=\cos(\frac{\pi}{6}+\frac{\pi}{4})=\cos(\frac{\pi}{6}).\cos(\frac{\pi}{4})-\sin(\frac{\pi}{6}).\sin(\frac{\pi}{4})=\frac{\sqrt{3}}{2} . \frac{1}{\sqrt{2}} - \frac{1}{2} . \frac{1}{\sqrt{2}}=\frac{\sqrt{3}-1}{2\sqrt{2}}=\frac{\sqrt{6}-\sqrt{2}}{4}$Đáp án:

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