Gaokao Math Practice Question
已知向量 $\boldsymbol{a}, \boldsymbol{b}$ 满足 $|a|=5,|b|=6, \boldsymbol{a} \cdot b=-6$, 则 $\cos \langle\boldsymbol{a}, \boldsymbol{a}+\boldsymbol{b}\rangle=(\qquad)$
- A.$-\frac{31}{35}$
- B.$-\frac{19}{35}$
- C.$\frac{17}{35}$
- D.$\frac{19}{35}$
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