A company makes solid cylinders of variable radius r cm and constant volume 128pi cm^3, It's surface area is 256π/r + 2πr^2. Find the minimum value for the surface area of the cylinder

Reply 7

standardolo

Original post by munishab

r u sure its not dy/dr?

Original post by the bear

the difficulty is that this is a function of r, not x.

A company makes solid cylinders of variable radius r cm and constant volume 128pi cm^3, It's surface area is 256π/r + 2πr^2. Find the minimum value for the surface area of the cylinder

Reply 8

standardolo

Original post by DFranklin

Post the full question.

A company makes solid cylinders of variable radius r cm and constant volume 128pi cm^3, It's surface area is 256π/r + 2πr^2. Find the minimum value for the surface area of the cylinder

Reply 9

Interea

Original post by standardolo

A company makes solid cylinders of variable radius r cm and constant volume 128pi cm^3, It's surface area is 256π/r + 2πr^2. Find the minimum value for the surface area of the cylinder

So you want to find d/dr(256π/r + 2πr^2)? Remember that pi is a constant, and then it's just basic differentiation of r^-1 and r^2 with constant multipliers.

Reply 10

standardolo

Original post by Interea

So you want to find d/dr(256π/r + 2πr^2)? Remember that pi is a constant, and then it's just basic differentiation of r^-1 and r^2 with constant multipliers.

Yeah i got that for 2πr^2 (4πr) but for 256π/r idk how to do that one, do i have to rearrange or something

Reply 11

DFranklin

Original post by standardolo

What's dy / dx of 256π/r + 2πr^2

Please tell me how thx

DFranklin

Post the exact question

Original post by standardolo

That's the question xD

Later...

Original post by standardolo

A company makes solid cylinders of variable radius r cm and constant volume 128pi cm^3, It's surface area is 256π/r + 2πr^2. Find the minimum value for the surface area of the cylinder

So, it wasn't the exact question, huh?

I have to ask: what's the mentality of making the "That's the question xD" reply? If people ask you to post the exact question, it's because they know there's something wrong or missing with what you originally posted.

Reply 12

davros

Original post by standardolo

What's dy / dx of 256π/r + 2πr^2

Please tell me how thx

OK,

grammar point: you don't "find dy/dx of " anything - you either find dy/dx, if y is given as a function of x, or you find the derivative of some function with respect to some other independent variable :smile:

In this case you have a function f(r) and you want to find df/dr (sometimes written as f'(r)).

I suspect what's holding you up is that 256pi/r is the same as (256pi)r^-1 and I'm sure you know a rule for differentiating r^-1 with respect to r :smile: