Let $\bar{a}, \bar{b}, \bar{c}$ be three vectors such that $|\bar{a}|=|\bar{c}|=1,|\bar{b}|=4,|\bar{b}…
Let $\bar{a}, \bar{b}, \bar{c}$ be three vectors such that $|\bar{a}|=|\bar{c}|=1,|\bar{b}|=4,|\bar{b} \times \bar{c}|=\sqrt{15}$. If $\lambda \bar{a}=\bar{b}-2 \bar{c}$, then the value of $\lambda$ is