If A = [0, - tan ((θ)/2); tan ((θ)/2), 0] and ( I_2+ A )( I_2- A )^-1= [a, - b; b, a], then 13( a^2+ b^2) is equal to \ \ \ \.

If $\displaystyle \mathrm{A}=\left[\begin{array}{cc}0 & -\tan \left(\frac{\theta}{2}\right) \\ \tan \left(\frac{\theta}{2}\right) & 0\end{array}\right]$ and $\displaystyle \left(\mathrm{I}_2+\mathrm{A}\right)\left(\mathrm{I}_2-\mathrm{A}\right)^{-1}=\left[\begin{array}{cc}\mathrm{a} & -\mathrm{b} \\ \mathrm{b} & \mathrm{a}\end{array}\right]$, then $\displaystyle 13\left(\mathrm{a}^2+\mathrm{b}^2\right)$ is equal to \_\_\_\_.