Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Determinants homework question

  1. Dec 18, 2008 #1
    determinants and the positive and negetive parts off the equation.

    ive had a couple say they change when they are worked out.

    on a second order process off mulitiplying diagonally

    1 (-1 -11) - -3(1 -11) + -3(2 -1)
    (1 5) (3 5) (3 1 )

    they are joined together and matrics above


    1(-5+11) +3(10 +33)-3(2+3)
    = 6+3(43)-3(5)
    =6+129-15=135-15=120

    ANs 120

    +\ mulitiply -1*5 and the -/ mulitpy1*11 making the 1(-5+11) would it not be 1-(5 -11) as the rule imforms you.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Dec 18, 2008 #2

    HallsofIvy

    User Avatar
    Science Advisor

    Re: determinants

    It's very difficult to figure out what you are trying to say here.
    I think you mean that you want to find the determinant
    [tex]\left|\begin{array}{ccc}1 & -3 & -3\\2 & -1 & -11\\3 & 1 & 5\end{array}\right|[/tex]
    (that "1" in "(1 -11)" is a typo, isn't it?)

    Yes, that is correct.

    What rule? 1(-5+ 11)= (-1)(5- 11) since 1(6)= (-1)(-6) but 1-(5-11) means 1 subtract 11 which is certainly wrong.
     
  4. Dec 18, 2008 #3
    Re: determinants

    in the matrics

    -1 *5 is a positve multiplication and 1 *-11 is a negitive mulitiplication accross the matric would this not effect the answer in the bracets or would allways be 1(-5+11)

    im still trying to work out how to write up on here the brackets for a matice but thank you for your help.
     
  5. Dec 18, 2008 #4

    HallsofIvy

    User Avatar
    Science Advisor

    Re: determinants

    I don't understand what you mean by a "positive multiplication" or a "negative multiplilcation". a(b+ c)= ab+ bc no matter what a, b, and c are.
     
  6. Dec 18, 2008 #5
    Re: determinants

    in the book im studying it has a diagonal mulityplication system ie were 1 and -11 are the top row and 1 and 5 are the bottom row.

    so top right is mulitiplyed with bottom left. do you see what i mean and what i wanted to know was there a main reason that or what made the addition or subtraction as a result off the mulitiply in the 1(-5+11)
     
  7. Dec 18, 2008 #6
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook