How Do You Solve This First Order Linear ODE?

In summary, to solve the first order linear ODE x (dy/dx)-2y=6x5, the integrating factor should be 1/x^2. After dividing through by x, the equation becomes dy/dx -2y/x = 6x^4. The integrating factor of e^-2ln(x) simplifies to e^ln(x^2) = x^2. After using the integrating factor, the equation becomes d/dx(-2y) = -12x^5/5 + C, which simplifies to -2y = -12x^5/5 + C. The final solution should be y = (-6/5)x^5 + 2C. The
  • #1
charmedbeauty
271
0

Homework Statement



Solve this first order linear ODE

x (dy/dx)-2y=6x5

Homework Equations


The Attempt at a Solution

x (dy/dx)-2y=6x5

divide through by x

dy/dx -2y/x = 6x4

I= e∫-2/x dx

=e-ln(2) =-2

∴ -2(dy/dx) + 4y/x = -12x4

d/dx(-2y) = -12x5/5 + C

y= 6/5(x5) - C/2

this is wrong but where did I go wrong?

Thanks
 
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  • #2
You didn't calculate the integrating factor correctly. Recheck the integral.
 
  • #3
vela said:
You didn't calculate the integrating factor correctly. Recheck the integral.

ohh it should be a half; thanks

so it should be y = y=-6/5x5 - 2C

the answer in my book has 2x5 + Ax2
 
  • #4
No, it should have an x in it.

EDIT: It refers to the integrating factor.
 
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  • #5
charmedbeauty said:
ohh it should be a half; thanks

so it should be y = y=-6/5x5 - 2C

the answer in my book has 2x5 + Ax2

The book is correct. Show your steps and we can help you find where you went astray.
 
  • #6
charmedbeauty said:
=e-ln(2) =-2

This isn't true; take a look at log/exponential rules.
 
  • #7
Bohrok said:
This isn't true; take a look at log/exponential rules.

okbthanks I figured it out now../.

I=1/x2

although I am alittle confused about this integral

if you work out the integral of -2/x

-2ln(x)

so I thought e^-2ln(x)

which is e^1/-2ln(x)


should be -1/2x

and not -1/x^2

??

thanks.
 
  • #8
You seem to be making one algebra mistake after another. Yes, the integral of -2/x is -2ln(x) and the exponential of that is [itex]e^{-2\ln(x)}[/itex] but that is NOT [itex]e/(-2\ln(x))[/itex]. I can't imagine where you got that. It is, rather [itex]1/e^{2\ln(x)}[/itex].

But what you should have done first was simplify -2 ln(x). Do you know how to take that -2 'into' the logarithm?
 
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  • #9
HallsofIvy said:
You seem to be making one algebra mistake after another. Yes, the integral of -2/x is -2ln(x) and the exponential of that is [itex]e^{-2\ln(x)}[/itex] but that is NOT [itex]e/(-2\ln(x))[/itex]. I can't imagine where you got that. It is, rather [itex]1/e^{2\ln(x)}[/itex].

But what you should have done first was simplify -2 ln(x). Do you know how to take that -2 'into' the logarithm?


yeah I made a stupid mistake of course e-2ln(x) = 1/e2ln(x)

its been a while so first I should of done

-2ln(x) = -ln(x2)

Thanks
 
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FAQ: How Do You Solve This First Order Linear ODE?

What is a first order linear ODE?

A first order linear ordinary differential equation (ODE) is an equation that relates a function and its derivative in the form of:
a(x)y'(x) + b(x)y(x) = c(x).
Where a(x), b(x), and c(x) are functions of x.

What is the general solution to a first order linear ODE?

The general solution to a first order linear ODE is in the form of:
y(x) = Ce-∫a(x)dx + ∫c(x)e∫a(x)dxdx.
Where C is an arbitrary constant.

What is the difference between a first order linear ODE and a non-linear ODE?

A first order linear ODE has a linear relationship between the function and its derivative, while a non-linear ODE has a non-linear relationship. This means that the coefficients in a linear ODE are constants, while in a non-linear ODE, they may be functions of x.

How do you solve a first order linear ODE?

To solve a first order linear ODE, you can use the method of integrating factors, which involves multiplying both sides of the equation by an integrating factor to make the left side a perfect derivative. You can also use separation of variables, where you separate the variables on each side of the equation and integrate both sides.

What are some real-world applications of first order linear ODE's?

First order linear ODE's are commonly used in the fields of physics, engineering, and economics to model various phenomena such as heat transfer, population growth, and electrical circuits. They are also used in various other fields to model real-world situations and make predictions.

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