# Thread: optimization: cylinder inscribed in a sphere

1. ## optimization: cylinder inscribed in a sphere

Find the dimensions of the right circular cylinder of maximum volume that can be inscribed in a sphere of radius a

so for the main equation that we will differentiate, i determined that
V(of cylinder) = (pi)(r^2)(h)

and for the connector, i made the picture of the spere and the cylinder flat so that it was two-dimensional and it became a rectangle inscribed in a circle.

i looked at this picture and decided that i had to somehow connect the cylinder to the sphere so i decided to use the equation of a circle as a connector(was this right?)

connector: r^2= x^2 + y^2
from there i found that y=(r^2-x^2)^(1/2) for later use

after this i dont even know where i went....i think i accidentally mixed up some x's and y's that i labeled before and got an illegitimate answer. so basically im confused.

2. V = pi*r<sup>2</sup>h

h = 2(R<sup>2</sup> - x<sup>2</sup>)<sup>1/2</sup>

V = 2pi*x<sup>2</sup>(R<sup>2</sup> - x<sup>2</sup>)<sup>1/2</sup>

now ... find dV/dx and find the value of x that maximizes the volume, then find h.

3. Originally Posted by skeeter
V = pi*r<sup>2</sup>h

h = 2(R<sup>2</sup> - x<sup>2</sup>)<sup>1/2</sup>where does the 2 come from? if you derived this equation of h from the equation of a circle, where could the 2 come from?

V = 2pi*x<sup>2</sup>(R<sup>2</sup> - x<sup>2</sup>)<sup>1/2</sup>

now ... find dV/dx and find the value of x that maximizes the volume, then find h.

4. also, isnt the x that we are using for the radius of the cylinder and the x we are using in the equation of the height two different x's?

5. center a circle of radius R at the origin.

inscribe a rectangle inside the square such that its sides are parallel to the coordinate axes.

rotate the rectangle and circle about the y-axis ... you get a cylinder inscribed in a sphere.

horizontal base of the rectangle has a length = 2x ... that would form the diameter of the cylinder, so the radius of the cylinder = x

vertical distance from the x-axis to the circle along the vertical side of the rectangle is y = (R<sup>2</sup> - x<sup>2</sup>)<sup>1/2</sup>

the total vertical distance, or height of the cylinder, is twice this distance ...
h = 2y = 2(R<sup>2</sup> - x<sup>2</sup>)<sup>1/2</sup>

now ... substitute those expressions, all in terms of x, into the cylinder volume equation.

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