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Unformatted text preview: Unit 15 Matrix Operations Recall that: A = ( a ij ) = a 11 a 12 a 13 ·· · a 1 n a 21 a 22 a 23 ·· · a 2 n a 31 a 32 a 33 ·· · a 3 n . . . . . . . . . . . . . . . a m 1 a m 2 a m 3 ·· · a mn is said to be an m × n matrix, where m × n is called the dimension of A . That is, a matrix whose dimension is m × n has m rows and n columns. and also that: the ( i, j )entry of A , denoted by a ij , is the entry (i.e., number) in row i and column j of matrix A . That is, for the ( i, j )entry, i is the row number, and j is the column number, where the rows are numbered 1 to m , from the top down, and the columns are numbered 1 to n , from left to right. Definition 15.1. Let A be any m × n matrix, and let k and t be integers with 1 ≤ k ≤ n and 1 ≤ t ≤ m . We use a k to denote the k th column of matrix A , and a t to denote the t th row of matrix A . For instance, for A as shown above, we have: a 2 = a 12 a 22 a 32 . . . a m 2 , a 3 = bracketleftbig a 31 a 32 a 33 · ·· a 3 n bracketrightbig Notice: In a k , all entries have the same second subscript k (because all are in the same column), while in a t , all entries have the same first subscript t (because all are in the same row). 1 Definition 15.2. Two matrices, A and B , are equal , written A = B , if they have the same dimension and their corresponding entries are equal. That is, A and B are equal if both are m × n matrices (for the same values of m and n ) and if a ij = b ij for each i and j . Example 1 . State whether matrices A and B are equal. (a) A = bracketleftbigg 1 2 3 4 6 bracketrightbigg B = bracketleftbigg 1 2 3 4 6 bracketrightbigg (b) A = bracketleftbigg 1 0 3 5 1 2 bracketrightbigg B = bracketleftbigg 1 0 3 5 1 2 bracketrightbigg (c) A = bracketleftbigg 1 2 3 4 5 6 bracketrightbigg B = 1 4 2 5 3 6 (d) A = bracketleftbigg 1 0 0 1 bracketrightbigg B = 1 0 0 1 0 0 Solution: (a) A = bracketleftbigg 1 2 3 4 6 bracketrightbigg = B = bracketleftbigg 1 2 3 4 6 bracketrightbigg Since A and B both have dimension 2 × 3 and a ij = b ij for each pair ( i, j ), they are equal matrices. (b) A = bracketleftbigg 1 0 3 5 1 2 bracketrightbigg negationslash = B = bracketleftbigg 1 0 3 5 1 2 bracketrightbigg Although A and B are both 2 × 3 matrices, with many of their entries iden tical, there is a combination ij for which a ij negationslash = b ij ( i.e. a 23 = 2 whereas b 23 = 2). Therefore, A and B are not equal matrices. (c) A = bracketleftbigg 1 2 3 4 5 6 bracketrightbigg negationslash = B = 1 4 2 5 3 6 Here, A has dimension 2 × 3, whereas B has dimension 3 × 2, so they cannot be equal matrices, no matter how similar their entries may be....
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This note was uploaded on 11/03/2009 for the course MATH 0110A taught by Professor Olds,vicky during the Fall '09 term at UWO.
 Fall '09
 OLDS,VICKY
 Calculus, Matrix Operations

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