The graph between object distance u and image distance v for a lens...

From the lens formula: $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ we have, $\frac{1}{f}=\frac{1}{10}-\frac{1}{-10}$ or $f=+5$ Further, $\Delta u=0.1$ and $\Delta v=0.1$ (from the graph) Now, differentiating the lens formula we have, $\frac{\Delta f}{f^2}=\frac{\Delta v}{v^2}+\frac{\Delta u}{u^2} \text{ or } \Delta$f=\left(\frac{\Delta v}{v^2}+\frac{\Delta u}{u^2}\right)$f^2 Substituting the values we have,$\Delta$f=\left(\frac{0.1}{10^2}+\frac{0.1}{10^2}\right)($5)^2 \Rightarrow f $\pm \Delta$f=5 $\pm$0.05