Register to reply

Derivative by Leibniz's integral rule

by Undoubtedly0
Tags: derivative, integral, leibniz, rule
Share this thread:
Feb8-13, 08:12 PM
P: 94
Hi all. I am looking at the functions [itex]\tau(x,y)[/itex], [itex]T(x,y)[/itex], and [itex]R(T)[/itex] related by

[tex] \tau(x,y) = \int_{\pi}^{T(x,y)} R(\theta) d\theta .[/tex]

It seems that by Leibniz's integral rule

[tex] \frac{\partial \tau}{\partial x} = \int_\pi^{T} \frac{\partial R}{\partial x} d\theta + R(T)\frac{\partial T}{\partial x}. [/tex]

It seems that [itex]\partial R / \partial x [/itex] need not be zero, yet another resource tells me that

[tex] \frac{\partial \tau}{\partial x} = R(T)\frac{\partial T}{\partial x} .[/tex]

Have I gone wrong somewhere? Thanks!
Phys.Org News Partner Science news on
New model helps explain how provisions promote or reduce wildlife disease
Stress can make hard-working mongooses less likely to help in the future
Grammatical habits in written English reveal linguistic features of non-native speakers' languages
Feb8-13, 08:51 PM
HW Helper
P: 2,263
Sometimes ∂R/∂x is zero, sometimes not. Here you have written R(θ) which is often a hint that ∂R/∂x is zero, otherwise one usually writes R(θ,x).
Feb8-13, 09:16 PM
P: 94
[itex]\theta[/itex] is a dummy variable that stands for [itex]T = T(x,y)[/itex], which I think means that [itex]R(\theta) = R(\theta(x,y))[/itex], correct?

Feb9-13, 12:55 AM
HW Helper
P: 2,263
Derivative by Leibniz's integral rule

$$\dfrac{d}{dt}\int^{\mathrm{b}(t)}_{\mathrm{a}(t)} \mathrm{f}(x,t) \, \mathrm{dx}=\int^{\mathrm{b}(t)}_{\mathrm{a}(t)} \dfrac{\partial \mathrm{f}}{\partial t} \, \mathrm{dx}+\mathrm{f}(\mathrm{b}(t),t) \mathrm{b} ^ \prime (t)-\mathrm{f}(\mathrm{a}(t),t) \mathrm{a} ^ \prime (t)$$

No the dummy variable takes all the values in a set like (pi,T), it does not depend on the end point. The other term gives the effect of the boundary. Leibniz's integral rule breaks the dependence of the integral on the variable into dependence on the the interior and the boundary. Usual examples are cars on a highway or water in a pipe or electricity in a wire.
Feb9-13, 08:20 AM
P: 94
So if we take it to be the case that

[tex] \frac{\partial \tau}{\partial x} = R(T)\frac{\partial T}{\partial x},[/tex]

then what would be the value of [itex] \partial \tau/\partial x |_{x=c} [/itex]?

[tex] \left.\frac{\partial \tau}{\partial x}\right|_{x=c} = R(T)|_{x=c} \cdot\left.\frac{\partial T}{\partial x}\right|_{x=c}[/tex]

What is meant by [itex] R(T)|_{x=c}[/itex]?
Feb9-13, 02:45 PM
elfmotat's Avatar
P: 260
Quote Quote by Undoubtedly0 View Post
What is meant by [itex] R(T)|_{x=c}[/itex]?

Let's say you're given the point (a,b). Plug that point into T and it returns a number (call it c): T(a,b)=c. Then you plug this number into R to get R(T): R(T(a,b))=R(c)=d.

If you're only given x=a but y remains a variable, then T(a,y) is a function dependent only on the variable y. Thus R(T) is also a function dependent only on y. Plugging in a particular value of y will return a single number for R(T).

Register to reply

Related Discussions
Leibniz's Rule Proof With Definition of a Derivative Calculus & Beyond Homework 2
Commuting derivative/Integral (not FTC or Leibniz) Calculus 2
Leibniz' Integral rule Calculus & Beyond Homework 2
Leibniz Rule (derivative of an integral) Calculus 0
Leibniz integral rule proof ? General Math 2