若非零向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$满足 $\left|\stackrel{\rightharpoonup }{a}\right|=3\left|\stackrel{\rightharpoonup }{b}\right|=|\stackrel{\rightharpoonup }{a}+2\stackrel{\rightharpoonup }{b}|$，则 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$夹角的余弦值为.