Turn on thread page Beta
    • Thread Starter
    Offline

    19
    ReputationRep:
    (the highlighted part) I don't know how to find (+x+2) the rest I got correct..where did I mess up? I know the " x " and " 2b " was missed out but it was for a reason

    Name:  answer2.jpg
Views: 18
Size:  215.4 KB
    Attachment 695884695886
    Attachment 695884695886696000
    [attach]6.958846958866959e+23[/attach]
    [attach]6.958846958866958e+29[/attach]
    Attached Images
        
    Offline

    12
    ReputationRep:
    (Original post by ckfeister)
    (the highlighted part) I don't know how to find (+x+2) the rest I got correct..where did I mess up? I know the " x " and " 2b " was missed out but it was for a reason
    I haven't looked through this in great detail, and this may be what you mean when you talk about the 2b thing, but shouldn't your trial function be of the form a cos(2x) + b sin(2x) + cx + d? That extra constant might make all the difference when you differentiate twice and substitute back in.
    Offline

    21
    ReputationRep:
    (Original post by ckfeister)
    (the highlighted part) I don't know how to find (+x+2) the rest I got correct..where did I mess up? I know the " x " and " 2b " was missed out but it was for a reason

    Name:  answer2.jpg
Views: 18
Size:  215.4 KB
    Attachment 695884695886
    Attachment 695884695886696000
    [attach]6.958846958866959e+23[/attach]
    [attach]6.958846958866958e+29[/attach]
    If you let y=a\sin 2x +b\cos 2x + cx+d and then solve for all 3 constants I believe you'll get your answer. I might be wrong though. Try it out and see what you get.
    Offline

    12
    ReputationRep:
    (Original post by ckfeister)
    (the highlighted part) I don't know how to find (+x+2) the rest I got correct..where did I mess up? I know the " x " and " 2b " was missed out but it was for a reason
    I've seen it through now, and it is the lack of a constant term in your trial function that is the problem. Try it again with one there.
    • Thread Starter
    Offline

    19
    ReputationRep:
    (Original post by Pangol)
    I've seen it through now, and it is the lack of a constant term in your trial function that is the problem. Try it again with one there.
    but wouldn't d=0 as there is no constant on the other side?
    Offline

    20
    ReputationRep:
    if you have x on the right then you need to include ax + b in your Particular Integral
    Offline

    12
    ReputationRep:
    (Original post by ckfeister)
    but wouldn't d=0 as there is no constant on the other side?
    If you include d from the beginning, you end up with a constant term on the LHS of d - 2c. It is this that is equal to zero, and since c is not zero, neither is d.

    In general, of you have a quadratic of degree n on the RHS, then your trial function also has to include a quadratic of degree n. There is one exception to this - if the differential equation only has terms in the second and first derivative, and no term in y, you instead need to use a quadratic of degree n+1 (although this time, curiously, it does not have to have a constant term!).
    • Thread Starter
    Offline

    19
    ReputationRep:
    (Original post by Pangol)
    I've seen it through now, and it is the lack of a constant term in your trial function that is the problem. Try it again with one there.
    your right, thx

    (Original post by Desmos)
    If you let y=a\sin 2x +b\cos 2x + cx+d and then solve for all 3 constants I believe you'll get your answer. I might be wrong though. Try it out and see what you get.
    it was cx+d
    Offline

    11
    ReputationRep:
    Two things to point out:

    - As others have said, in your trial function, you need the term: cx + d,
    so your trial function should be:

     y(x) = a \sin(2x) + b \cos(2x) + cx + d, (where a, b, c and d are constants).

    - You are using b twice in your trial function, as you are using (as your trial function):

     y(x) = a \sin(2x) + b \cos(2x) + b.
 
 
 

University open days

  • University of Chichester
    Multiple Departments Undergraduate
    Thu, 25 Oct '18
  • Norwich University of the Arts
    Undergraduate Open Days Undergraduate
    Fri, 26 Oct '18
  • University of Lincoln
    Open Day Undergraduate
    Sat, 27 Oct '18
Poll
Who do you think it's more helpful to talk about mental health with?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Equations

Best calculators for A level Maths

Tips on which model to get

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.