Giải hệ pta) $\left\{ \begin{array}{l} 5xy=6(x+y)\\ 7yz=12(y+z)\\3xz=4(x+z) \end{array} \right.$b)$\left\{ \begin{array}{l} \frac{xyz}{x+y}=\frac{24}{5}\\ \frac{xyz}{y+z}=\frac{24}{7}\\\frac{xyz}{x+z}=4\end{array} \right.$c)$\left\{ \begin{array}{l} x+y+z=1\\ x+2y+4z=8\\x+3y+9z=27 \end{array} \right.$