The sum of the surface areas of a rectangular parallelepiped with sides x, 2x and and a sphere is given to be constant. Prove that the sum of their volumes is minimum, if x is equal to three times the radius of the sphere. Also find the minimum value of the sum of their volumes.


Given: a rectangular parallelepiped with sides x, 2x and . The sum of the surface areas of a rectangular parallelepiped and a sphere is given to be constant.


To prove: the sum of their volumes is minimum, if x is equal to three times the radius of the sphere and also find the minimum value of the sum of their volumes


Now the surface area of the rectangular parallelepiped is




Now let the radius of the sphere be r cm, then the surface area of the sphere is,


Ss=4πr2


Now sum of the surface areas of a rectangular parallelepiped and a sphere becomes,





Now given the sum of the surface areas of a rectangular parallelepiped and a sphere is constant, then



So differentiating equation (i) with respect to r, we get



Applying differentiation rule of sum, we get



Taking out the constant terms, we get



Applying the derivative, we get







Let V denotes the sum of volume of the rectangular parallelepiped and a sphere, then




For maxima or minima, the first derivative of volume should be equal to 0, i.e.,



So differentiating equation (iii) with respect to r, we get



Applying differentiation rule of sum, we get



Taking out the constant terms, we get



Applying the derivative, we get




Now substituting the value of from equation (ii), we get







i.e., x is three times the radius of the sphere when sum of the volumes of rectangular parallelepiped and sphere


Hence proved


Now to find the minimum volume of the sum of their volumes we need to find out the second derivative value at x=2r.


Hence applying derivative with respect to r to equation (iv), we get



Applying differentiation rule of sum, we get



Taking out the constant terms, we get



Applying differentiation rule of product to the first part, we get




Now substituting the value of from equation (ii), we get




Now substituting x=3r, we get






This is positive; hence V is minimum when x=3r or , and the minimum value of volume can be obtained by substituting in equation (iii), we get






Is the minimum value of the sum of their volumes.


34
1