matrix inverse of a 3 by 3 matrix

what is the inverse of the matrix 1 3 -2
-3 0 -5
2 5 0

3 Answers

30
Ashish Kothari ·

|A|= \begin{vmatrix} 1 & 3&-2 \\ -3& 0 &-5 \\ 2& 5 &0 \end{vmatrix}

Do the following operations, R2→R2+3R1 and R3→R3-2R1 and expand by C1,

= \begin{vmatrix} 1 & 3 & -2\\ 0& 9 & -11 \\ 0& -1& 4 \end{vmatrix} = (36-11)=25

A^{-1}=\frac{adjA}{|A|}

adjA=\begin{pmatrix} A_{11} & A_{12} &A_{13} \\ A_{21} & A_{22} & A_{23} \\ A_{31} & A_{32} & A_{33} \end{pmatrix}^{T}

adjA=\begin{pmatrix} 25 & -7 & -17 \\ -13 & 5 & 1 \\ -13 & 11 & 10 \end{pmatrix}^{T}

A^{-1}=\frac{1}{25}\begin{pmatrix} 25 & -13 & -13 \\ -7 & 5 & 11 \\ -17 & 1 & 10 \end{pmatrix}

edits: I forgot to do the transpose.

1
saikat_iit2012sa SAMANTA ·

can u do it by elementary transformations only

11
Devil ·

A simple method by elementary transformations -

Form the matrix B= \left(A : I_3 \right)

Now apply elementary row/col. operations to make the mat. A the Identity matrix. Apply the corresponding operations to I. Then you'll be left with (I:A^{-1})

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