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: Express as a linear combination

  1. Mar 26, 2008 #1
    1. The problem statement, all variables and given/known data
    The vectors v1= [1, -3], v2= [2, -8], v3= [ -3, 7] span R2 but do not form a basis. Find 2 different ways to express [1, 1] as a linear combination of v1, v2, v3.

    3. The attempt at a solution

    Since it states that the set is not a basis, then v3=v1+v2. I solved the system v1+v2=[1,1] and got 5v1-2v2=[1,1]. The second answer the book gives is 10v1-3v2+v3. How did they get the second answer?
  2. jcsd
  3. Mar 26, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    If you solve c1*v1+c2*v2=[1,1] you get a unique solution since [v1,v2] are a basis. To get the second answer you solve c1*v1+c2*v2+c3*v3=[1,1]. Since [v1,v2,v3] are linearly dependent, and hence are not a basis, you get an infinite number of solutions. The books answer is only one of them.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook