Q.The numerical value of the volume and the lateral surface area of a right circular cone are equal. If the height of the cone is \( h \) and the radius is \( r \), determine the value of \(\cfrac{1}{h^2}+\cfrac{1}{r^2}\).

The slant height of the cone \((l)=\sqrt{h^2+r^2}\)

According to the given condition, \(\cfrac{1}{3} πr^2 h=πrl\)
Or, \(\cfrac{1}{3} rh=l\)
Or, \(l=\cfrac{1}{3} rh\)
Or, \(\sqrt{h^2+r^2}=\cfrac{1}{3} rh\)
Or, \(h^2+r^2=\cfrac{1}{9} r^2 h^2\)
Or, \(\cfrac{h^2+r^2}{r^2 h^2}=\cfrac{1}{9}\)
Or, \(\cfrac{h^2}{r^2 h^2}+\cfrac{r^2}{r^2 h^2}=\cfrac{1}{9}\)
Or, \(\cfrac{1}{r^2} +\cfrac{1}{h^2} =\cfrac{1}{9}\)

∴ \(\cfrac{1}{h^2} +\cfrac{1}{r^2} =\cfrac{1}{9}\) (Answer)
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