[tex]\frac{1}{f} = \frac{1}{u} + \frac{1}{v}[/tex]
multiply f to the entire equation
[tex]1= \frac{f}{u}+\frac{f}{v} [/tex]
add up right side
[tex]1=\frac{fv}{uv} +\frac{fu}{uv}[/tex]
[tex]1= \frac{fv+fu}{uv}
[/tex]
multiply uv to both sides
[tex]uv= fv+fu[/tex]
factor out f
[tex]uv= f(v+u)[/tex]
divide (v+u) to both sides
[tex]\frac{uv}{v+u}=f[/tex]
substitute in u=3.5cm and v=-4 cm
