Find x,y and z if 1/x+1/y=1995,1/y+1/z=1996 dhe 1/x+1/z=1997
Use the substitution
$\displaystyle u=\dfrac1x$ and $\displaystyle v=\dfrac1y$ and $\displaystyle w=\dfrac1z$
Then you have to solve the system of simultaneous equations:
$\displaystyle \left|\begin{array}{rccccl}u&+&v&&=&1995 \\ &v&+&w&=&1996 \\u&+&w&& =& 1997\end{array}\right.$
Afterwards re-substitute to calculate x, y, z.