$$\left. \begin{array} { l } { a b + a c = 71 , a + b + c = ? } \\ { a c - b c = 40 } \end{array} \right.$$
$a\neq 0\text{, }b=\frac{71-ac}{a}\text{, }c=\frac{a\sqrt{a^{4}+18a^{2}+5041}-|a|a^{2}+71|a|}{2a|a|}\text{, }d=\frac{a^{2}+71}{a}$
$a\neq 0\text{, }b=\frac{71-ac}{a}\text{, }c=\frac{-a\sqrt{a^{4}+18a^{2}+5041}-|a|a^{2}+71|a|}{2a|a|}\text{, }d=\frac{a^{2}+71}{a}$
