已知 $\left|\stackrel{\rightharpoonup }{a}\right|=\left|\stackrel{\rightharpoonup }{b}\right|=2$， $(\stackrel{\rightharpoonup }{a}+2\stackrel{\rightharpoonup }{b})·(\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b})=-2$，则 $\stackrel{\rightharpoonup }{a}$与 $\stackrel{\rightharpoonup }{b}$的夹角为.