Korzystamy z równania zwierciadła:
[tex]\frac{1}{f} = \frac{1}{x}+\frac{1}{y}\\\\\frac{1}{x} = \frac{1}{f}-\frac{1}{y}\\\\\frac{1}{x} = \frac{y}{fy} - \frac{f}{fy}\\\\\frac{1}{x} = \frac{y-f}{fy}\\\\\frac{1}{\frac{1}{x}} = \frac{1}{\frac{y-f}{fy}}\\\\\boxed{x = \frac{fy}{y-f}}[/tex]
