设 $\left(\begin{array}{cccc}\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{6}} & -\frac{1}{\sqrt{12}} & \frac{1}{2} \\ \frac{1}{\sqrt{2}} & -\frac{1}{\sqrt{6}} & \frac{1}{\sqrt{12}} & -\frac{1}{2} \\ 0 & \frac{2}{\sqrt{6}} & \frac{1}{\sqrt{12}} & -\frac{1}{2} \\ 0 & a & b & \frac{1}{2}\end{array}\right)$ 为正交矩阵, 则 $a=$ ,$b=$
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