# In any $\Delta ABC,a(b \cos C-c \cos B)=$

$\begin {array} {1 1} (1)\;b^2+c^2 & \quad (2)\;b^2-c^2 \\ (3)\;\frac{1}{b}+\frac{1}{c} & \quad (4)\;\frac{1}{b^2}-\frac{1}{c^2} \end {array}$