1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Value of combinaison of orthogonal elements

  1. Oct 23, 2013 #1
    Hello this is not a homework, just studying for the exam, and :
    1. The problem statement, all variables and given/known data
    Consider E a linear space with dot product (.,.) and the norm ||x|| = sqrt(x,x)
    a and b two orthogonals elements of E
    Find the value of ||a+ b|| et||a- b|| and ||a+ b||-||a- b||

    2. Relevant equations

    3. The attempt at a solution
    a and b orthogonals means that ||a||=||b||=1
    ||a+b||=√(a+b,a+b) = √[(a,a)+(b,a)+(a,b)+(b,b) ] =√[ 1+0+0+1 ] = √2
    ||a-b||=√(a-b,a-b) = √[(a,a)+(-b,a)+(a,-b)+(-b,-b) ] =√[ 1-(b,a)+(-b,a)*-1 ] = √[ 1-(b,a)-(b,a)*-1 ]=0

    Is that correct ?
  2. jcsd
  3. Oct 23, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If ||b|| = 1, then (-b,-b) = 1, not -1.

    Also orthogonal typically just means that (a,b) = 0, without giving any restriction on the size of the vector (if they are all unit vectors they're typically called orthonormal), so you should probably double check the wording of the problem/your class's definition of orthogonal
  4. Oct 24, 2013 #3
    They're orthonormal, I just made a mistake copying the problem statement.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted