设$0<\theta <\frac{\pi }{2}$，向量$\stackrel{\rightharpoonup }{a}=(\sin 2\theta ,\cos \theta ),\stackrel{\rightharpoonup }{b}=(\cos \theta ,1)$，若$\stackrel{\rightharpoonup }{a}\parallel \stackrel{\rightharpoonup }{b}$，则$\tan \theta$=.