已知向量 $a,b$满足 $(\stackrel{\rightharpoonup }{a}+2\stackrel{\rightharpoonup }{b})·(\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b})=-6$，且 $\left|\stackrel{\rightharpoonup }{a}\right|=1$, $\left|\stackrel{\rightharpoonup }{b}\right|=2$,则 $a$与 $b$的夹角为.