1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Eliminating constants

  1. Oct 17, 2012 #1
    1. The problem statement, all variables and given/known data

    eliminating c1 and c2 from y, y' and y'' to create a differential equation

    2. Relevant equations

    I have the equation y = (c1 +c2x)e^x
    y' = c2e^x +(c1+c2x)e^x
    y'' = c2e^x + c2e^x + (c1+c2x)e^x)

    (from product rule)

    3. The attempt at a solution

    I'm just not sure how to eliminate the constants. If I try to do y'' - y i still have constants left.
     
  2. jcsd
  3. Oct 17, 2012 #2

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    What do you get if you plug y = (c1 +c2x)ex for the second term in y' ?

    Solve that for c2.
     
  4. Oct 17, 2012 #3
    with y"-y you are only using part of you available information.
    What happens if you subtract y-y' = ... ?
    or y'-y" ?
     
  5. Oct 17, 2012 #4
    thank you! I can't believe I didn't see that.

    I got c2 = (y' - y)/ e^x. I can then sub this into y'' to get the differential.

    Thank you :)
     
    Last edited: Oct 17, 2012
  6. Oct 17, 2012 #5
    Isn't it c2 = (y' - y) e^-x ?
     
  7. Oct 17, 2012 #6
    yes yes, for some reason it didn't put the / button
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook