3．Given that \(x > y > 0\) ，
（a）by writing \(\log _ { y } x = z\) ，or otherwise，show that \(\log _ { y } x = \frac { 1 } { \log _ { x } y }\) ．
（b）Given also that \(\log _ { x } y = \log _ { y } x\) ，show that \(y = \frac { 1 } { x }\) ．
（c）Solve the simultaneous equations $$\begin{gathered} \log _ { x } y = \log _ { y } x
\log _ { x } ( x - y ) = \log _ { y } ( x + y ) \end{gathered}$$