Q. 234.8( 4 Votes )

Show that the rig

Answer :

Let the given volume be V and radius of base of cone be r and h be the height and l be the slant height of cone

Let S be the surface area

We have to find h such that the surface area is minimum so we have to establish a equation for S in terms of h

The surface area is given by S = πrl

Where l is slant height of cone given by

We need to eliminate r

For that given is volume of cone V

Square equation (a)

S2 = π2r2(r2 + h2)

Put value of r2

Now to find for what value of h S is minimum we first need to find the critical points such that

Differentiate S with respect to h and equate to 0

0 = -18V2 + 3Vh3π

0 = -6V + h3π

h3π = 6V

Now we have to check whether is point of minima or maxima

If then h is a point of minima

Let us check

From (i)

Differentiate again with respect to h

The LHS will be differentiated using the uv rule which states that (uv)’ = u’v + uv’ where u = 2S and

Substitute value of from (i)


Now observe that S represents surface area V represents volume which are measures for some quantity hence both are positive π is a positive constant hence and hence is a point of minima

Now let us find h in terms of r by putting the value of volume


h2 = 2r2

h = √2r

Hence for least curved surface area the altitude h has to be √2 times the base radius

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