# Math Help - Evaluating parametic derivative

1. ## Evaluating parametic derivative

u = 3t-1 t = v2-7v w = 7/v ; find du/dw at w= -7

I tried solving for du/dt and got 3
dt/dv = 2v-7 and dw/dv = -7/v^2

I'm not sure how the w = -7 is supposed to work with this because if i just plug it in to the third equation i get a v value which i don't need

I already know that du/dw is essentially du/dt / dw/dt. I have du/dt.

Question: How do I solve for dw/dt? Any help appreciated, thanks!

2. ## Re: Evaluating parametic derivative

\displaystyle \begin{align*} \frac{du}{dw} = \frac{du}{dt} \cdot \frac{dt}{dv} \cdot \frac{dv}{dw} \end{align*}.

Notice that \displaystyle \begin{align*} w = \frac{7}{v} \implies v = \frac{7}{w} \end{align*}.

3. ## Re: Evaluating parametic derivative

Thanks so much!!!