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# If C is a 5x1 matrix and D is a 3x5 matrix, what are the dimensions of DC?

is it 3x1

### 3 Answers by Expert Tutors

Katherine P. | Experienced Math and Test Prep TutorExperienced Math and Test Prep Tutor
5.0 5.0 (572 lesson ratings) (572)
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The formula you want to remember (or write in your notes) is:

( m x n ) x ( n x p ) = ( m x p )

The columns of the first matrix (n) need to equal the rows of the second matrix (n).

DC = (3 x 5) x ( 5 x 1)

Applying ( m x n ) x ( n x p ) = ( m x p ), you get the dimensions of DC:

( 3 x 1)

Hope that helps!

Alan B. | National Board Certified TeacherNational Board Certified Teacher
5.0 5.0 (5 lesson ratings) (5)
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Remember that matrices have dimensions listed in row,column order so your Matrix C is 5X1 (meaning 5 rows and 1 column) and your Matrix D is 3X5 (meaning 3 rows and 5 columns). When you have an operator between the two matrices, for example D X C, you arrange the dimensions in that same order. In your case, 3X5 x 5x1 and the middle two (columns of the first matrix and rows of the second matrix) MUST match for the operation to be possible. Your resulting matrix will have the rows of the first matrix and the columns of the second, in this case 3x1   Hope this helped!

Brian B. | Making Math Matter...and Easy to Understand! Making Math Matter...and Easy to Underst...
5.0 5.0 (226 lesson ratings) (226)
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Yes....its a 3x1, but make sure you order the matrix as 3x5 times 5x1  the middle numbers have to match...columns from the first match the rows of the second.