### Theory:

A diameter of the wafer cone is $$10 \ cm$$ and, its height is $$12 \ cm$$. Calculate the curved surface area of $$20$$ such wafer cones.

Solution:

Diameter of the cone $$(d)$$ $$=$$ $$10 \ cm$$

Radius of the cone $$(r)$$ $$=$$ $\frac{d}{2}=\frac{10}{2}=5$ $$cm$$

Height of the cone $$(h)$$ $$=$$ $$12 \ cm$$

Let us first find the slant height of the cone.

$$l = \sqrt{r^2 + h^2}$$

$$l = \sqrt{5^2 + 12^2}$$

$$l = \sqrt{25 + 144}$$

$$l = \sqrt{169}$$

$$l = 13$$ $$cm$$

Curved surface area of the cone $$=$$ $$\pi r l$$ sq. units

$$=$$ $\frac{22}{7}×5×13$

$$=$$ $\frac{1430}{7}$

$$=$$ $$204.28$$ $$cm^2$$

Curved surface area of a cone is $$204.28 \ cm^2$$.

Curved surface area of $$20$$ cones:

$$=$$ $$20 \times 204.28$$

$$=$$ $$4085.6$$

Therefore, the curved surface area of $$20$$ wafer cones is $$4085.6 \ cm^2$$.

Important!
The value of $$\pi$$ should be taken as $$\frac{22}{7}$$ unless its value shared in the problem.