Using Chain Rule to Solve Questions - A Step-by-Step Guide

In summary, the chain rule can be used to solve equations in which different variables are functions of one another.
  • #1
EconometricAli
4
0
Hey everyone, could anyone help me figure out how to use chain rule to solve these questions in the attachments below?
 

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  • #2
Okay your second picture gives the "chain rule":
If f is a function of x, y, and z and x, y, and z are functions of the variables, s and t, then f is a function of s and t and $\frac{\partial f}{\partial t}= f_x\frac{\partial x}{\partial t}+ f_y\frac{\partial y}{\partial t}+ f_z\frac{\partial z}{\partial t}$ and $\frac{\partial f}{\partial s}= f_x\frac{\partial x}{\partial s}+ f_y\frac{\partial y}{\partial s}+ f_z\frac{\partial z}{\partial s}$.
Great!

Now, in the first problem, $f(x, y, z)= xyz^{10}$, $x= t^3$, $y= ln(s^2\sqrt{t})$, and $z= e^{st}$.

The first thing I would do is write $y= 2(1/2)ln(st)= ln(st)$.

Now $f_x= yz^{10}$ and $\frac{\partial x}{\partial t}= 3t^2$ so $f_x\frac{\partial x}{\partial t}= 3t^2yz^{10}$. If you want to reduce that to s and t only, replace y with $ln(st)$ and z with $e^{st}$ to get $f_x\frac{\partial x}{\partial t}= 3t^3ln(sy)e^{10st}$.

$f_y= xz^{10}$ and $\frac{\partial y}{\partial t}= \frac{s}{t}$1 so $f_y\frac{\partial y}{\partial t}= xz^{10}\frac{s}{t}=\frac{st^3e^{10st}}{t}= st^2e^{10t}$.

$f_z= 10xyz^9$ and $\frac{\partial z}{\partial t}=se^{st}$ so $f_z\frac{\partial z}{\partial x}= 10xyz^9(se^{st})= 10(t^3)ln(st)e^{9st}(se^{st})= 90st^3ln(st)e^{10st}$.

$\frac{\partial f}{\partial t}$ is the sum of those.

$\frac{\partial f}{\partial s}$ is done the same way but with $\frac{\partial x}{\partial s}$, $\frac{\partial y}{\partial s}$, and $\frac{\partial z}{\partial s}$.
 
  • #3
Country Boy said:
The first thing I would do is write $y=2(1/2)ln(st)=ln(st)$
$y = \ln(s^2 \cdot \sqrt{t}) = 2\ln(s) + \dfrac{1}{2}\ln(t)$
 
  • #4
One of these days I really need to learn Math!
 
  • #5
Thank you for the response but I'm lost in part b. The portion where fx (dx/dt) for x^10+y^11+z^10... what is its final form?
 
  • #6
Okay, "part b" has $f= x^{10}+ y^{11}+ z$, $x= \sqrt{t+ s^2}$, $y=\frac{t}{s}$, $z= st$ and you want to find $\frac{\partial f}{\partial s}$ and $\frac{\partial f}{\partial t}$.

What are
$\frac{\partial f}{\partial x}$
$\frac{\partial f}{\partial y} $
$\frac{\partial f}{\partial z}$ ?

What are
$\frac{\partial x}{\partial s}$
$\frac{\partial x}{\partial t}$

$\frac{\partial y}{\partial s}$
$\frac{\partial y}{\partial t}$

$\frac{\partial z}{\partial s}$
$\frac{\partial z}{\partial t}$ ?

If you know basic Calculus you should be able to answer those.
If you can't do some of them, tell us which.

The "chain rule" says
$\frac{\partial f}{\partial s}= \frac{\partial f}{\partial x}\frac{\partial x}{\partial s}+ \frac{\partial f}{\partial y}\frac{\partial y}{\partial s}+ \frac{\partial f}{\partial z}\frac{\partial z}{\partial s}$
and
$\frac{\partial f}{\partial t}= \frac{\partial f}{\partial x}\frac{\partial x}{\partial t}+ \frac{\partial f}{\partial y}\frac{\partial y}{\partial t}+ \frac{\partial f}{\partial z}\frac{\partial z}{\partial t}$
 
  • #7
I got my answers marked and lost a lot of marks actually. I'm not sure why.
 

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FAQ: Using Chain Rule to Solve Questions - A Step-by-Step Guide

What is the chain rule and why is it important in solving questions?

The chain rule is a mathematical concept used to calculate the derivative of a composite function. It is important in solving questions because it allows us to find the rate of change of a function that is made up of multiple smaller functions.

How do I identify when to use the chain rule in a question?

The chain rule is used when the function being differentiated is a composition of two or more functions. Look for functions within functions, such as f(g(x)) or h(f(g(x))), to identify when the chain rule should be applied.

What are the steps for using the chain rule to solve a question?

The steps for using the chain rule are as follows:

  1. Identify the outer function and the inner function.
  2. Apply the power rule to the outer function.
  3. Multiply by the derivative of the inner function.
  4. Simplify the resulting expression.

Can the chain rule be used for functions with more than two nested functions?

Yes, the chain rule can be applied to functions with any number of nested functions. Each nested function will be treated as the inner function, and the chain rule will be applied successively until the final derivative is obtained.

Are there any common mistakes to avoid when using the chain rule?

One common mistake when using the chain rule is forgetting to apply the derivative to the inner function. Another mistake is not simplifying the resulting expression, which can lead to incorrect answers. It is also important to carefully identify the outer and inner functions in order to apply the chain rule correctly.

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