The altitude of the right circular cone of maximum volume that can be inscribed in a sphere of radius r is
A
r2
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B
r3
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C
3r4
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D
4r3
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Solution
The correct option is D4r3 Let R be the radius and h be the height of cone. ∴OA=h−r In ΔOAB, r2=R2+(h−r)2 ⇒r2=R2+h2+r2−2rh ⇒R2=2rh−h2 The volume V of the cone is given by V=13πR2h =13πh(2rh−h2)=13π(2rh2−h3) On differentiating with respect to h, we get dVdh=13π(4rh−3h2) For maximum and minimum, put dVdh=0 ⇒4rh=3h2 ⇒4r=3h ⇒h=4r3 .....(h≠0) Now, d2Vdh2=13π(4r−6h) AT h=4r3, (d2Vdh2)h=4r3=13π(4r−6×4r3) =π3(4r−8r) =−4rπ3<0 ⇒V is maximum when h=4r3. Hence, volume of the cone is maximum when h=4r3, which is the altitude of cone.