the derivatio of the formula of frustum of cone

Let ABC be a cone. A frustum DECB is cut by a plane parallel to its base. Let *r* _{1} and *r* _{2} be the radii of the ends of the frustum of the cone and *h* be the height of the frustum of the cone.

In ΔABG and ΔADF, DF||BG

∴ ΔABG ∼ ΔADF

CSA of frustum DECB = CSA of cone ABC − CSA cone ADE

CSA of frustum =

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