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Operations on Matrices>Scalar Multiplication

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Notes

For example , `3 xx [(2,4),(-5,1)]`
To get the result of matrix each element of  the matrix  multiply by 3.
The order of the above matrix is same as the order of the original matrix. The original matrix is  of the order `2 xx 2`  so the resulting matrix 
will also be of the same order.
`3 xx [(2,4),(-5,1)] = [(3 xx 2,3 xx 4),(3 xx-5 ,3 xx 1)]`
=`[(6 , 12),(-15 , 3)]`
Thus `k xx [(a,b),(c,d)]` = `[(ka , kb),(kc , kd)]`

Multiplying a matrix to a number is called scalar Multiplication.

Negative of matrix :
Z = `[(1,3),(-5,2)]`  -Z = ?
-Z = `-1 xx Z

=`-1 xx [(1,3),(-5,2)]`

=` [(1*-1 , 3*-1),(-5*-1, 2*-1)]`

=`[(-1 ,-3),(5, -2)] `

The negative of a matrix is denoted by –Z. We define –Z = (–1) Z.

Difference of matrices:
 If A = `[a_(ij)]`, B = `[b_(ij)]` are two matrices of the same order, say m × n, then difference A – B is defined as a matrix D = `[d_(ij)]`, where `d_(ij) = a_(ij) – b_(ij)`, for all value of i and j. In other words, D = A – B = A + (–1) B, that is sum of the matrix A and the matrix – B.
Video link : https://youtu.be/4lHyTQH1iS8

Notes

The matrix is the sum of the two matrices. The sum of two matrices is a matrix obtained by adding the corresponding elements of the matrices. 
Thus, if A = `[(a_11,a_12,a_13),(a_21,a_22,a_23)]` is a 2 x 3 matrix and

B = `[(b_11,b_12,b_13),(b_21,b_22,b_23)]` is another 2 x 3 matrix .

Then we define
A +B = `[(a_11 + b_11,a_12 + b_12,a_13 + b_13), (a_21 + b_21,a_22 + b_22,a_23 + b_23)]`

In general, if A = `[a_(ij)]` and B = `[b_(ij)]` are two matrices of the same order, say m × n. Then, the sum of the two matrices A and B is defined as a matrix C = `[c_(ij)]_(m × n)`, where `c_(ij)` = `a_(ij)` + `b_(ij)`, for all possible values of i and j.
Video link : https://youtu.be/ZCmVpGv6_1g

Notes

The product of two matrices A and B is defined if the number of columns of A is equal to the number of rows of B. Let A = `[a_(ij)]` be an m × n matrix and B = `[b_(jk)]` be an n × p matrix. Then the product of the matrices A and B is the matrix C of order m × p. To get the `(i, k)th`element cik of the matrix C, we take the ith row of A and kth column of B, multiply them elementwise and take the sum of all these products. In other words, if A = `[a_(ij)]_(m × n)`, B = `[b_(jk)]_(n × p)`, then the ith row of A is `[a_(i1)  a_(i2)  ...  a_("in")]` and the `k^(th)` column of

B is `[(b_(1k)) , (b_(2k)), (b_(nk))]` 
, then `c_(ik) =  a_(i1) b_(1k) + a_(i2) b_(2k) + a_(i3) b_(3k) + ... + a_("in") b_(nk)`   
=\[\displaystyle\sum_{j=1}^{n} a_{ij} b_{jk}\].

Non-commutativity of multiplication of matrices:
The below example that even if AB and BA are both defined, it is not necessary that AB = BA.
If A = `[(1,0),(0,-1)]` and B =`[(0,1),(1,0)]` ,

then AB `[(0,1),(-1,0)]`

and BA = `[(0,-1),(1,0)]`.
Clearly AB ≠ BA.
Thus matrix multiplication is not commutative.

Zero matrix as the product of two non zero matrices:
The real numbers a, b if ab = 0, then either a = 0 or b = 0. This need not be true for matrices, we will observe this through an example.
find AB ,if A = `[(0,-1),(0,2)]` and B = `[(3,5),(0,0)]`

We have AB = `[(0,-1),(0,2)][(3,5),(0,0)]`

=`[(0,0),(0,0)]`
Thus, if the product of two matrices is a zero matrix, it is not necessary that one of the matrices is a zero matrix.

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