como determinar la diagonal de un rectángulo ?

Usa el teorema de Pitágoras:

$${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{b}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

Por ejemplo:

16 es $${\mathtt{a}}$$ y 10 es $${\mathtt{b}}$$ . d es la diagonal $${\mathtt{c}}$$ .

$${{\mathtt{a}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{b}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

$${{\mathtt{16}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{10}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

$${\mathtt{256}}{\mathtt{\,\small\textbf+\,}}{\mathtt{100}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

$${\mathtt{356}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

$${\mathtt{c}} = {\sqrt{{\mathtt{356}}}}$$

$${\mathtt{c}} = {\sqrt{{\mathtt{356}}}} \Rightarrow {\mathtt{c}} = {\mathtt{18.867\: \!962\: \!264\: \!113\: \!207\: \!6}}$$