Related rates (parametric derivatives)

Question

inkyvoyd · Answer

if y=3x^2+2x and dx/dt=4, find dy/dt when x=3
My answer: dy/dt=24x+8; dy/dt=80.
Can someone please check this?
THanks!

tamtoan · Answer

isn't it dy/dt = 6x dx/dt + 2dx/dt ? can someone check this ?

inkyvoyd · Answer

:S

zepdrix · Answer

Yah @tamtoan is on the right track there,$$\huge y=3x^2+2x$$If we take the derivative with respect to TIME, we'll get,$$\huge \color{#CC0033}{\frac{dy}{dt}}=6x\color{#F35633}{\frac{dx}{dt}}+2\color{#F35633}{\frac{dx}{dt}}$$And they gave us information about dx/dt and x.$$\huge \color{#CC0033}{\frac{dy}{dt}}=6(3)(\color{#F35633}{4})+2(\color{#F35633}{4})$$Understand where those numbers are coming from?

inkyvoyd · Answer

Uh, the answer is defienitely 80, because I just submitted te assignment. Let me go over what happened though

inkyvoyd · Answer

Yeah, @zepdrix , you have the dx/dt because x is a function of t; @tamtoan didn't have that. Thanks.

zepdrix · Answer

Tam had the same thing, he just didn't plug in the x and dx/dt values for you :D

inkyvoyd · Answer

Oh he did :S. Well, thanks for helping out!