已知向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$夹角为 $45°$^ ，且 $\left|\stackrel{\rightharpoonup }{a}\right|=1,\left|2\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|=\sqrt{10}$；则 $\left|\stackrel{\rightharpoonup }{b}\right|=$.