Calculating the Inverse of AB: A^(-1)=[4,0;-2,2], B^(-1)=[-2,0;-2,3]

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Discussion Overview

The discussion revolves around finding the inverse of the product of two matrices, A and B, given their inverses. Participants explore the correct application of the formula for the inverse of a product of matrices and clarify the implications of matrix multiplication properties.

Discussion Character

  • Homework-related
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asserts that the inverse of the product AB is calculated as (AB)^(-1) = A^(-1) * B^(-1), leading to a specific numerical result.
  • Another participant counters that the correct formula is (AB)^(-1) = B^(-1) * A^(-1), emphasizing that matrix multiplication does not commute.
  • Some participants discuss the meaning of "commute" in this context and express confusion about the implications of non-commutativity.
  • One participant suggests calculating A and B from their inverses and then multiplying them to demonstrate the correct order of multiplication for finding the inverse.
  • A later reply confirms that the inverse of AB should indeed be B^(-1) * A^(-1) and acknowledges a mistake in the initial understanding of the problem.

Areas of Agreement / Disagreement

Participants disagree on the correct formula for the inverse of the product of matrices. While some support the initial claim, others provide corrections based on the properties of matrix multiplication. The discussion remains unresolved regarding the initial misunderstanding.

Contextual Notes

There are limitations in the understanding of matrix multiplication properties among participants, particularly regarding the non-commutative nature of matrix operations. Some participants express uncertainty about the implications of their calculations and the definitions involved.

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Homework Statement



Find the inverse of AB if A^(-1)= [4,0;-2,2] and B^(-1)=[-2,0;-2,3]. (See below for picture/additional information.)

Homework Equations



Inverse of AB = inverse of A*inverse of B

The Attempt at a Solution



Using above equation:

(AB)^(-1) = [4,0;-2,2]*[-2,0;-2,3] = [-8,0;0,6]

I don't understand why this is wrong. I calculated it by hand, and then used two different online matrix calculators when I was told it was wrong. The calculators agree with me. Am I entering it incorrectly? Here is a picture of the "full" question: http://imgur.com/foTsK2e.
Thanks.
 
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Inverse of AB = inverse of B*inverse of A
Matrix multiplication does not commute!
 
h.krish360 said:
Inverse of AB = inverse of B*inverse of A
Matrix multiplication does not commute!

Um, what does commute mean in this context?

EDIT: Looked it up, and I don't understand why you say that. So what if BA doesn't work (haven't even tested it - don't see how it is applicable).
 
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If you do the math to find A and B:

A = (A-1) -1

B = (B-1) -1

then multiply A and B, then take the inverse

(AB)-1

You'll find it's the same as (B-1) (A-1) and not the other way around. This is because matrix multiplicaion is associative, but not commutative (the next post has a link showing the math).
 
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rcgldr said:
If you do the math to find A and B:

A = (A-1) -1

B = (B-1) -1

then multiply A and B, then take the inverse

(AB)-1

You'll find it's the same as (B-1) (A-1) and not the other way around. This is because matrix multiplicaion is associative, but not commutative (the next post has a link showing the math).

So the inverse of AB should be B^(-1)*A^(-1)? Tried it: got the question right.

Thank you.

EDIT: My textbook got it right, I just didn't pay attention. Whoops...
 
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