- #1
bmanmcfly
- 45
- 0
Derivative with "constant" variables
Hi, I need some help figuring out this one situation that's got me thrown for a loop... not the first time, since my grasp of calculus is slippery at best.
a and b are both positive constants
Drag(D) = av^2+b/v^2
I'm trying to find the value of V that would represent a min for D
D=av^2+b/v^2
[tex]\frac{dD}{dv} = 2av-\frac{2b}{v^{3}}[/tex]
[tex]\frac{d^{2}D}{dv^{2}}= 2a+\frac{6b}{v^{4}}[/tex]
If I learned this proper, the derivative = 0 at Max / Min points... so;
[tex]0=2av-\frac{2b}{v^{3}}[/tex]
[tex]\frac{2b}{v^{3}}= 2a[/tex]
[tex]v=\left(\frac{b}{a}\right)^{-1/4}[/tex]
I figured that with the second derivative that since all positive values of v will have a positive value of d^2D/dv^2, and so any point of the derivative =0 will be a minimum point for D.
Now, since this is homework, I don't necessarily need the correct answer, but if I could be pointed to where I'm gong wrong (if I am)... I don't know why, but having these variables as constants is really throwing me for a loop, and any help / advice here would be appreciated.
Hi, I need some help figuring out this one situation that's got me thrown for a loop... not the first time, since my grasp of calculus is slippery at best.
Homework Statement
a and b are both positive constants
Drag(D) = av^2+b/v^2
I'm trying to find the value of V that would represent a min for D
Homework Equations
D=av^2+b/v^2
[tex]\frac{dD}{dv} = 2av-\frac{2b}{v^{3}}[/tex]
[tex]\frac{d^{2}D}{dv^{2}}= 2a+\frac{6b}{v^{4}}[/tex]
The Attempt at a Solution
If I learned this proper, the derivative = 0 at Max / Min points... so;
[tex]0=2av-\frac{2b}{v^{3}}[/tex]
[tex]\frac{2b}{v^{3}}= 2a[/tex]
[tex]v=\left(\frac{b}{a}\right)^{-1/4}[/tex]
I figured that with the second derivative that since all positive values of v will have a positive value of d^2D/dv^2, and so any point of the derivative =0 will be a minimum point for D.
Now, since this is homework, I don't necessarily need the correct answer, but if I could be pointed to where I'm gong wrong (if I am)... I don't know why, but having these variables as constants is really throwing me for a loop, and any help / advice here would be appreciated.
Last edited: