Gaokao Math Practice Question
已知向量 $\vec{a}=(2,1), \vec{a} \cdot \vec{b}=10,|\vec{a}+\vec{b}|=5 \sqrt{2}$, 则 $|\vec{b}|=$（$\qquad$）
- A.$\sqrt{5}$
- B.$\sqrt{10}$
- C.5
- D.25
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