Transcription of 2.5 Inverse Matrices - MIT Mathematics
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Inverse Inverse Matrices '&$%1If the square matrixAhas an Inverse , then bothA 1A=IandAA 1= test invertibility is elimination :Amust haven(nonzero) for invertibility is the determinant ofA:detAmust not be tests for invertibility isAx=0:x=0must be the only (same size) are invertible then so isAB:|(AB) 1=B 1A 1=Iisnequations forncolumns ofA 1. Gauss-Jordan eliminates[A I]to[I A 1].7 The last page of the book gives14equivalent conditions for a squareAto be a square matrix . We look for an Inverse matrix A 1of the same size,such thatA 1timesAequalsI. WhateverAdoes,A 1undoes. Their product is theidentity matrix which does nothing to a vector, soA 1Ax= 1might not a matrix mostly does is to multiply a vectorx. MultiplyingAx=bbyA 1givesA 1Ax=A isx=A 1b. The productA 1 Ais like multiplying bya number and then dividing by that number. A number has an Inverse if it is not zero Matrices are more complicated and more interesting.
To invert a 3 by 3 matrix A, we have to solve three systems of equations: Ax 1 = e 1 and Ax 2 = e 2 = (0,1,0) and Ax 3 = e 3 = (0,0,1). Gauss-Jordan finds A−1 this way. The Gauss-Jordan method computesA−1 by solving all n equations together. Usually the “augmented matrix” [A b] has one extra column b. Now we have three right sides e 1 ...
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