Related rate expanding rectangle

by synergix
Tags: expanding, rate, rectangle
 P: 178 1. The problem statement, all variables and given/known data The length of a rectangle is increasing at a rate of 8cm/s and its width is increasing at a rate of 3cm/s when the length is 20 cm and the width is 10cm, how fast is the area of the rectangle increasing? 2. Relevant equations V=LW 3. The attempt at a solution dL/dt=8 dW/dt=3 l=20 w= 10 dA/dt=? (V=LW)' dA/dt= dL/dt * dW/dt before I go any further is this correct both the L and the W become one when derived right?
 Sci Advisor HW Helper Thanks P: 25,244 A(t)=L(t)W(t), right? Take a look at the 'product rule'. And why would the derivative of L and W be 1???
P: 178
 Quote by Dick A(t)=L(t)W(t), right? Take a look at the 'product rule'. And why would the derivative of L and W be 1???
I have really just not been using my brain lately. They would be one if they were being added which they're not. (x + a)'= 2 does it not? if they are both variables.

And you have t in parenthesis to make clear that L and W and A change with time and are therefore not constants right?

so according to the product rule->

dA/dt=L(dW/dt)+W(dW/dt)

HW Helper
Thanks
P: 25,244
Related rate expanding rectangle

 Quote by synergix I have really just not been using my brain lately. They would be one if they were being added which they're not. (x + a)'= 2 does it not? if they are both variables. And you have t in parenthesis to make clear that L and W and A change with time and are therefore not constants right? so according to the product rule-> dA/dt=L(dW/dt)+W(dW/dt)
Ok, you've got the product rule. But (x+a)'=x'+a' if by ' you mean d/dt. Whether that's 2 or not depends what x and a are. dx/dx=1 but dx/dt doesn't necessarily equal 1.
 P: 178 Ok I was just using the power rule (x^1 + a^1)'= 1*x^0 + 1*a^0= 1+1 am I assuming something here I shouldn't be?
 Sci Advisor HW Helper Thanks P: 25,244 d(x(t)^1)/dt=1*x(t)^0*dx/dt, chain rule again.
 P: 178 Could you explain quickly why you need to use the chain rule here?
 Sci Advisor HW Helper Thanks P: 25,244 Because x(t) is a function of t. d(x^1)/dx is 1*x^0 (though you could still throw the chain rule in and write it as 1*x^0*dx/dx, but dx/dx=1). d(x^1)/dt=1*x^0*dx/dt. dx/dx is always 1. dx/dt is not.
P: 178
 Quote by Dick Because x(t) is a function of t. d(x^1)/dx is 1*x^0 (though you could still throw the chain rule in and write it as 1*x^0*dx/dx, but dx/dx=1). d(x^1)/dt=1*x*dx/dt. dx/dx is always 1. dx/dt is not.
Ok thank you its starting to come together.

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