• Support PF! Buy your school textbooks, materials and every day products Here!

Why is this matrix undefined?

  • Thread starter rocomath
  • Start date
  • #1
1,752
1
3D + E

D = 2 x 3

-1 2 3
4 0 5

E = 3 x 2

2 1
8 -1
6 5

D has 3 columns, and E has 3 rows ???
 
Last edited:

Answers and Replies

  • #2
1,752
1
Oh crap, ignore me ... LOL, I'm thinking inner product rules ... :p
 
  • #3
HallsofIvy
Science Advisor
Homework Helper
41,794
925
??? What matrix are you talking about? You give two matrices, D and E, both of which are defined because you just defined them.

The product DE is also defined but ED is not. Is that what you are talking about? Do I get a prize for guessing that?

The product of of two matrices, A and B can be defined as "the ij-component is the dot product of vectors consisting of the ith row of A and the jth column of B".

ED is not defined because each row of E has 2 components while each column of D has 3 components. You cannot take the dot product of two such vectors.

As you point out, the number of columns of D and the number of rows of E are the same- that is why DE is defined.
 
  • #4
1,752
1
Sorry Ivy! I misread the problem and kept thinking I was multiplying the two, the problem actually asks the addition of the two. I'm not actually doing the problem, just skimming through the section.
 
  • #5
cristo
Staff Emeritus
Science Advisor
8,107
72
I'm not actually doing the problem, just skimming through the section.
Perhaps less skimming and more 'doing' is in order! :wink:
 
  • #6
1,752
1
Perhaps less skimming and more 'doing' is in order! :wink:
LOL, I know I should be doing the problems :( But, I did the examples and looked over the rules. I plan on doing a a good review after finals :)
 

Related Threads for: Why is this matrix undefined?

  • Last Post
Replies
13
Views
3K
Replies
13
Views
817
  • Last Post
2
Replies
34
Views
4K
  • Last Post
Replies
6
Views
1K
  • Last Post
Replies
1
Views
2K
Replies
24
Views
2K
Replies
7
Views
837
Top