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## Homework Statement

I need to find an expression for the minimum drag coefficient, [tex]C_D[/tex], of an aircraft during straight and level flight.

## Homework Equations

[tex]C_D=C_{D_0}+kC_L^2[/tex]

Where:

[tex]C_{D_0}[/tex] is the profile drag coefficent

[tex]C_L[/tex] is the lift coefficient

[tex]k[/tex] is just a constant

## The Attempt at a Solution

I started off by plotting the equation for [tex]C_D[/tex] shown above and got the following:

http://g.imagehost.org/view/0218/drag [Broken]

So I thought that the minimum drag would be when:

[tex]\frac{dC_D}{dC_L}=0[/tex]

However, I remember my lecturer saying something about I would have to do:

[tex]\frac{d}{dC_L}\left(\frac{C_D}{C_L}\right)=0[/tex]

But I am unsure why, I was wondering if someone could shed some light on this please?

Thanks,

Ryan

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