Question 840705
* A shherical balloon is being inflated. Find the expression for the
instantaneuos rate of change of the volume with respect to the radius.
Evaluate this rate of change for the radius of 4.00m using the delta
process.
<pre>
The formula for the Volume is 
{{{V = (4/3)pi*r^3}}}
But we must NOT substitute 4 for r until after we have
found the derivative, since r is still a variable until 
we "freeze" r at the instant when r is 4.
I am using Dx for <font face ="symbol">D</font>x 
{{{matrix(2,2,
lim,     ((4/3)pi*(r+Dr)^3-(4/3)pi*r^3)/(Dr), 
"Dr->0" ,""

)}}} =
 
{{{matrix(2,2,
lim,     ((4/3)pi*((r+Dr)^3-r^3))/(Dr), 
"Dr->0" ,""

)}}} =
{{{matrix(2,2,
lim,     ((4/3)pi*((r^3+3r^2*Dr+3r*Dr+(Dr)^3)-r^3))/(Dr), 
"Dx->0" ,""

)}}} =
{{{matrix(2,2,
lim,     ((4/3)pi*(r^3+3r^2*Dr+3r*Dr+(Dr)^3-r^3))/(Dr), 
"Dx->0" ,""

)}}} =
{{{matrix(2,2,
lim,     ((4/3)pi*(3r^2*Dr+3r*Dr+(Dr)^3))/(Dr), 
"Dx->0" ,""

)}}} =
{{{matrix(2,2,
lim,     ((4/3)pi*Dr(3r^2+3r*Dr+(Dr)^2))/(Dr), 
"Dx->0" ,""

)}}} =
{{{matrix(2,2,
lim,     ((4/3)pi*cross(Dr)(3r^2+3r*Dr+(Dr)^2))/(cross(Dr)), 
"Dx->0" ,""

)}}} = 

{{{matrix(2,2,
lim,     ((4/3)pi(3r^2+3r*Dr+(Dr)^2)), 
"Dx->0" ,""

)}}} =
{{{(4/3)pi(3r^2+3r*0+(0)^2)}}} =
{{{(4/3)pi(3r^2)}}} =
{{{(4/cross(3))pi(cross(3)r^2)}}} =
{{{4pi*r^2}}}
Finally, we substitute r = 4
{{{4pi*(4)^2}}} =
{{{4pi*16}}} =
{{{64*pi}}}
</pre>
find the derivative of the implicit function [3x^2
divided by  (y^2 + 1) plus y= 3x + 1]
{{{(3x^2)/(y^2+1)+y = 3x+1}}}<pre>
Use the quotient formula for the first expression
Remember that although the derivative of x is 1, the
derivative of y is NOT 1 but {{{(dy)/(dx)}}}
{{{( (y^2+1)(6x)-(3x^2)(2y*expr(dy/dx)))/(y^2+1)^2+(dy)/(dx) = 3}}}

Multiply through by the denominator (y²+1)²

{{{ (y^2+1)(6x)-(3x^2)(2y*expr(dy/dx))+(y^2+1)^2*expr((dy)/(dx)) = 3(y^2+1)^2}}}

{{{6xy^2+6x-6x^2y*expr(dy/dx)+(y^2+1)^2*expr((dy)/(dx))=3(y^2+1)^2}}}

{{{-6x^2y*expr(dy/dx)+(y^2+1)^2*expr((dy)/(dx))=3(y^2+1)^2-6xy^2-6x}}} 

{{{(-6x^2y+(y^2+1)^2)*expr((dy)/(dx))=3(y^2+1)^2-6xy^2-6x}}} 

{{{(dy)/(dx)=(3(y^2+1)^2-6xy^2-6x)/(-6x^2y+(y^2+1)^2)}}} 
 
That's as far as I'm taking it.  You can multiply all that out
if you like.  There is no sense in any teacher giving you a
problem that comes out so messy and unwieldy as that!

Edwin</pre>