Solution: The angle between the diagonal of a square and its side is $45^\circ$, but here the angle is $150^\circ$, which lies in the second quadrant. Using the reference angle $30^\circ$, $\cos 150^\circ = -\cos 30^\circ = -\frac{\sqrt{3}}{2}$. $\boxed{-\dfrac{\sqrt{3}}{2}}$

Mon, 04 May 2026 10:56:57 +0000

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