Integrating a complicated polynomial

In summary, the student was struggling to integrate with respect to r and substituted u = 1+r^2 but that didn't work. Finally, they discovered that trigonometric substitution was the answer and were stuck.
  • #1
Mathpower
28
0

Homework Statement


Hi
An integration question:
t= 1/(1+r2)

Can you please show me how to integrate with respect to r. Thank you.



The Attempt at a Solution


∴t = (1+r2)-1
I then tried substituting u = 1+r^2 but that didn't work!
Is there a trick with this?
 
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  • #2
Well I'll hint and say it has to do with an inverse trig function.
 
  • #3
Just so I understand, you're trying to find the integral of this?
[tex]\int \frac{1}{1+r^2}\,dr[/tex]
BTW, what you have after the t is not a polynomial, but a rational expression.
Mathpower said:

The Attempt at a Solution


∴t = (1+r2)-1
I then tried substituting u = 1+r^2 but that didn't work!
Is there a trick with this?
Yes, it's called trigonometric substitution. Try r = tan u.
 
  • #4
eumyang...thank you...yes that's what i meant.
 
  • #5
NO idea! I tried r= tan u
1/(1+(tan u)^2)...
Too me that just seems even more worse!

Then (LET: < be integral sign/ integrand)
[itex]
= <1/(1+u) dr
= <(1/(1+u)) (secu)^2 du
= <((secu)^2/(1+u)) du [/itex]
Stuck...

and my latex is not working as well? Have I typed the wrong code?
 
Last edited:
  • #6
Mathpower said:
NO idea! I tried r= tan u
1/(1+(tan u)^2)...
Too me that just seems even more worse!

Then (LET: < be integral sign/ integrand)
[itex]
= <1/(1+u) dr
= <(1/(1+u)) (secu)^2 du
= <((secu)^2/(1+u)) du [/itex]
Stuck...
No no, put the two attempts together:
[tex]\int \frac{1}{1+r^2}\,dr = \int \frac{1}{1+\tan^2 u}\,\sec^2 u \, du[/tex]
You know your trig identities, right?

EDIT: Regarding LaTeX, that's because you're using the wrong tags. It should be tex or itex in the brackets, not latex.
 
  • #7
Delete this. See below
 
Last edited:
  • #8
Oh wait is the answer:
u because 1+tan^2 u = sec^2 u
then
[tex] = ∫1 du
=u + C [/tex]
=tan(r)^-1 + C

Thank you for your help.
 
Last edited:
  • #9
Mathpower said:
oh wait is the answer u because 1+tan^2 u = sec^2 u
then <1
If you mean
[tex]\int 1[/tex]
...then you're missing something.
Mathpower said:
=u
=tan(r)^-1 + C
Do you mean
[itex]\tan r^{-1} + C[/itex]
or
[itex]\tan^{-1} r + C[/itex]?
 
  • #10
Opps... I meant [tex] tan-1 r+C [/tex]

Thank you so much for your help. Sorry for being lazy.
 

1. What is a complicated polynomial?

A complicated polynomial is an algebraic expression that contains multiple variables, exponents, and coefficients, making it difficult to simplify or solve by traditional methods.

2. What does it mean to integrate a polynomial?

Integrating a polynomial is the process of finding its antiderivative or the original function from which the polynomial was derived. This can be used to calculate the area under a curve or solve certain types of equations.

3. How do I integrate a complicated polynomial?

Integrating a complicated polynomial involves using a combination of algebraic techniques, such as substitution and integration by parts, along with knowledge of basic integration rules and properties. It may require multiple steps and careful manipulation to arrive at the final answer.

4. What are the benefits of integrating a polynomial?

Integrating a polynomial can provide valuable insights into the behavior and relationships of a function. It can also be useful in solving real-world problems involving rates of change and optimization.

5. Are there any shortcuts for integrating a complicated polynomial?

While there are some general strategies and techniques for integrating polynomials, there is no one-size-fits-all shortcut for integrating a complicated polynomial. The best approach is to carefully analyze the polynomial and apply appropriate integration methods as needed.

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