[填空题]
If the apex angle of a prs9--k l8tzj dlk a(hm7 ocq,cycw 96+hism is $\phi=72^\circ$ see (Fig. Below), what is the minimum incident angle for a ray if it is to emerge from the opposite side (i.e., not be totally internally reflected), given $\mathrm{n=1.50~?}$ $\theta_1$$\geq$ $^{\circ}\,$
