1. ## Chain rule help

Can someone explain to me how we get the last line from the chain rule please?

Thank you

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2. ## Re: Chain rule help

You have:

$\dfrac{dz}{d\tau} = \dfrac{dz}{d(1-\tau)}\dfrac{d(1-\tau)}{d\tau}$

The derivative of $1-\tau$ with respect to $\tau$ is -1, so the second term is -1.

3. ## Re: Chain rule help

Originally Posted by SlipEternal
You have:

$\dfrac{dz}{d\tau} = \dfrac{dz}{d(1-\tau)}\dfrac{d(1-\tau)}{d\tau}$

The derivative of $1-\tau$ with respect to $\tau$ is -1, so the second term is -1.
Thanks! How come the first term doesn't change at all though? I thought with chain rule you have to take derivative of both functions?

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4. ## Re: Chain rule help

You are "taking the derivative of both functions". $\frac{dz}{d(1- \tau)}$ is the first derivative and $\frac{d(1- \tau)}{d\tau}= -1$ is the second derivative.