Q.Let A=[3−456] and B=[−912−7], find AB and BA.
Puducherry CbseNCERTSubjective· 3mImportance★★★★★
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Concept understanding — Matrix Equality
Matrix Equality: When Two Grids Are Truly the Same
Think of a matrix as a spreadsheet or a grid of numbers. Two matrices are equal only when they are identical in every possible way — same shape, same numbers in the same positions.
The Intuition
Imagine you have two seating charts for a classroom. Each chart is a grid where every seat has a student's name. When would you say the two charts are the same?
First, both charts must have the same number of rows and columns — you can't compare a 3×4 chart with a 2×6 chart.
Second, for every seat position, the name written there must match exactly between the two charts.
That's matrix equality in a nutshell.
The Precise Definition
Two matrices A and B are said to be equal, written A=B, if and only if:
They have the same order (same number of rows m and same number of columns n).
For every position (i,j), the corresponding entries are equal: aij=bij for all 1≤i≤m and 1≤j≤n.
A=B⟺order(A)=order(B) and aij=bij for all i,j
What This Means in Practice
If A=(1324) and B=(1324), then A=B — they are the same matrix.
But if C=(142536) and D=135246, then C=D because C is 2×3 while D is 3×2. Different shape means different matrix, even if the numbers are the same.
Watch out
A common mistake is to think that two matrices are equal just because they contain the same set of numbers. The positions matter just as much as the values. (1324) and (1234) are not equal — the 2 and 3 have swapped places.
Why This Matters
Matrix equality is the foundation for solving matrix equations. When you see something like:
(x32y)=(5327)
you can immediately conclude that x=5 and y=7, because equality forces every corresponding entry to match. This is how you solve for unknown variables inside matrices — you set up a system of equations by equating entry by entry.
In short: Two matrices are equal only when they are carbon copies — same dimensions, same numbers in the same places. Nothing less counts.
Multiplying the two matrices in each order, row by column, gives AB and BA separately, and comparing the two products shows whether matrix multiplication commutes here.
✓Final answer
AB=[−2242−29−50] and BA=[−3531−33−37] (so AB=BA).
Multiply row-by-column for both orders; the products differ, showing matrix multiplication is not commutative.
For 2×2 matrices, (PQ)ij=∑kpikqkj — each entry is (row i of the first)⋅(column j of the second).
Compute AB with A=[3−456], B=[−912−7]:
(1,1):3(−9)+5(1)=−27+5=−22,
(1,2):3(2)+5(−7)=6−35=−29,
(2,1):−4(−9)+6(1)=36+6=42,
(2,2):−4(2)+6(−7)=−8−42=−50.
AB=[−2242−29−50].
Compute BA:
(1,1):−9(3)+2(−4)=−27−8=−35,
(1,2):−9(5)+2(6)=−45+12=−33,
(2,1):1(3)+(−7)(−4)=3+28=31,
(2,2):1(5)+(−7)(6)=5−42=−37.
BA=[−3531−33−37].
Compare:AB=BA, confirming matrix multiplication is generally non-commutative.
✓Final answer
AB=[−2242−29−50],BA=[−3531−33−37], and AB=BA.