For a concave mirror, if real image is formed the graph between $\frac{1}{\mathrm{u}} \mathrm{and} \frac{1}{\mathrm{v}}$is of the form

$$\begin{gathered} \mathrm{since} \frac{1}{\mathrm{f}}=+\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}}\Rightarrow \frac{1}{\mathrm{v}}=-\frac{1}{\mathrm{u}}+\frac{1}{\mathrm{f}} \mathrm{Putting} \mathrm{the} \mathrm{sign} \mathrm{convention} \mathrm{properly} \\ \frac{1}{\left(-\mathrm{v}\right)}=\frac{-1}{\left(-\mathrm{u}\right)}+\frac{1}{\left(-\mathrm{f}\right)}\Rightarrow \frac{1}{\mathrm{v}}=-\frac{1}{\mathrm{u}}+\frac{1}{\mathrm{f}} \\ \mathrm{comparing} \mathrm{this} \mathrm{equation} \mathrm{with} \mathrm{y}=\mathrm{mx}+\mathrm{c} \mathrm{slope} = \mathrm{m}= \mathrm{tan\theta }=-1\Rightarrow {135}^{\mathrm{o}} \mathrm{or} -{45}^{\mathrm{o}} \mathrm{and} \mathrm{intercept} \mathrm{C}=+\frac{1}{\mathrm{f}} \end{gathered}$$