the derivatio of the formula of frustum of cone
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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 =