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!

Matrices in Matlab

  1. Jan 29, 2007 #1
    Question 1:

    Solve the system of equations

    -w + 3x - 2y + z = 1
    w + 2x - 4y - 3z = 2
    x + 2y + z = 4
    2w + 3x - y - z = 7

    The attempt at a solution

    To do this in matlab, this is what I will did:

    >> A=[-1,3,-2,1;1,2,-4,-3;0,1,2,1;2,3,-1,-1]; b = [1;2;4;7]
    >>x=inv(A)*b

    Then matlab should give an answer with the solutions of w, x, y and z written in a separate line…no?

    Question 2:

    Find the eigenvalues and eigenvectors of the matrix

    3, -1, 0, 0
    -1, 5, -2, 0
    0, -2, 6, -1
    0, 0, -1, 2

    The attempt at a solution

    In matlab do I do this:

    >>A=[3,-1,0,0;-1,5,-2,0;0,-2,6,-1;0,0,-1,2] ;
    >>[V,D]=eig(A)

    Then I should get an answer for V and D, and v is a matrix whose columns are the eigenvectors and D is the diagonal matrix, so we multiply the first column of V with the first diagonal of D etc… am I wrong?

    Question 3:

    Let

    A=

    -3, 4, 2, 0
    1, -2, 0, 1
    -4, 5, 0, 3
    0, 1, -4, 2

    Write the Matlab commands to construct the following matrix:

    3, 0, 0, 0, -3, 1, -4, 0
    0, 4, 0, 0, 4, -2, 5, 1
    0, 0, 5, 0, 2, 0, 0, -4
    0, 0, 0, 6, 0, 1, 3, 2
    -3, 4, 2, 0, 5, 5, 5, 5
    1, -2, 0, 1, 5, 5, 5, 5
    -4, 5, 0, 3, 5, 5, 5, 5
    0, 1, -4, 2, 5, 5, 5, 5

    The attempt at a solution

    In matlab should i do this:

    >>A=[-3,4,2,0;1,-2,0,1;-4,5,0,3;0,1,-4,2]

    Then to construct the other big matrix do I do this:

    >> B=[diag(3:6),A’;A, fives(4,4)]

    You’ve probably all figured that I don’t have access to matlab just now, so I would be grateful if someone who has access to matlab could just copy and paste the commands in to see if it works.

    Also between each question do I write clear all to clear everything before I start the question?
     
    Last edited: Jan 29, 2007
  2. jcsd
  3. Jan 29, 2007 #2

    J77

    User Avatar

    For question 2, you don't have to do anything after eig.

    D is a diagonal matrix of eigenvalues - just read them off the diagonal, and V is the corresponding matrix of eigenvectors - just take each column separately.

    (You also have the relationship A*V=V*D - this is what you may be thinking when you want to multiply...?)

    Qu 3 is right but for, there is no such thing as fives: use 5*ones(4,4).

    Qu 1 looks good too.
     
    Last edited: Jan 29, 2007
  4. Jan 29, 2007 #3
    ok thanx
    do you know if i did Question 3 right coz i have a feeling it is wrong?
     
  5. Jan 29, 2007 #4

    J77

    User Avatar

    See edit above for Qu3.
     
  6. Jan 29, 2007 #5
    ok thanx i really appreciate your help!

    and you didn't have to say 'there is no such thing' just correct me lol
     
  7. Jan 29, 2007 #6

    J77

    User Avatar

    That's my new found Dutch abruptness invading my English politeness :tongue:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Matrices in Matlab
  1. Matrices in matlab (Replies: 2)

Loading...