A cone of height 3 cm is formed by folding the given figure.

What is the base radius of the cone so formed?

It can be seen that when the given figure is folded, its radius becomes the slant height of the cone.

∴ Slant height, l = 5 cm

Height, h = 3 cm

We know that,

$\begin{array}{rcl}{\mathrm{l}}^2& =& {\mathrm{h}}^2+{\mathrm{r}}^2\\ & \Rightarrow & {\mathrm{r}}^2={\mathrm{l}}^2-{\mathrm{h}}^2\\ & =& 5^2-3^2\\ & =& 25-9\\ & =& 16\\ & \Rightarrow & \mathrm{r}=4\end{array}$

Therefore, the base radius of the cone so formed is 4 cm.