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Problem type

Inverse of a matrix by row reduction

What it solves
Find the inverse of a 2 by 2 or 3 by 3 matrix by reducing the matrix beside the identity, with every row operation worked entry by entry, then prove the answer by multiplying back. If the matrix has no inverse, see exactly where and why the method stops.
Subject
Linear Algebra
Topic
Determinants and inverses
Also called
invert a matrix, matrix inverse, Gauss Jordan
Your problem

Find the inverse of this matrix, or show that it has none.

A=[130−1]A = \left[ \begin{array}{rr} 1 & 3 \\ 0 & -1 \end{array} \right]
The square matrix
Size2
Whole numbers or fractions. 2 by 2 or 3 by 3.
Invent a new one:
Femtobytes
Inverse of the 2 by 2 matrix 1 3 / 0 -1 by row reduction, step by step
femtobytes.com/solve/matrix-inverse/1c3r0c-1
The problem

Find the inverse of this matrix, or show that it has none.

A=[130−1]A = \left[ \begin{array}{rr} 1 & 3 \\ 0 & -1 \end{array} \right]
The result
A−1=[130−1]A^{-1} = \left[ \begin{array}{rr} 1 & 3 \\ 0 & -1 \end{array} \right]

Checked: multiplying AA by this matrix gives the identity.

checked 18 lines of this solution were evaluated with exact arithmetic before it was shown to you. A solution with a false line is never served.
Picture 1 of 2Step 1: Say what an inverse is, and set up the method
A written beside the identity, which is the whole method
1133110000−1-10011
On the left the matrix A, on the right the identity, the matrix that changes nothing. Every move made to the left half is made to the right half too. When the left half has become the identity, the right half has become the inverse, because it has had exactly the moves done to it that undo A.
1

Say what an inverse is, and set up the method

Why this step

The inverse of AA is the matrix that UNDOES AA. We could hunt for its entries as unknowns, which for a 3×33 \times 3 matrix means nine unknowns in nine equations. There is a far better idea: every row operation is itself a multiplication by some matrix, so the whole sequence of operations that turns AA into the identity IS, multiplied together, the inverse. We do not need to find those matrices. We only need to apply the same operations to the identity and watch what it turns into.

[13100−101]\left[ \begin{array}{rr|rr} 1 & 3 & 1 & 0 \\ 0 & -1 & 0 & 1 \end{array} \right]
Write AA and II side by side. We will row reduce the whole block. When the left half becomes II, the right half will have become A−1A^{-1}.
A written beside the identity, which is the whole method
1133110000−1-10011
On the left the matrix A, on the right the identity, the matrix that changes nothing. Every move made to the left half is made to the right half too. When the left half has become the identity, the right half has become the inverse, because it has had exactly the moves done to it that undo A.
2

Pivot 1: column 1, row 1

Why this step

We work one column at a time, left to right. The goal for a column is a single 11 (the pivot) with zeros everywhere else in it. A column in that state says something very plain: its variable appears in exactly one equation, with nothing in front of it. Each cleared column is one variable pinned down.

The pivot is already 11 and the rest of its column is already zero, so there is nothing to do for this column.

[13100−101]\left[ \begin{array}{rr|rr} \boxed{1} & 3 & 1 & 0 \\ 0 & -1 & 0 & 1 \end{array} \right]
The matrix as it stands, with this pivot boxed. checked
Column 1: the pivot, and what it has to clear
1133110000−1-10011
the pivot: the entry being used the entries it is driving to zero the row the pivot lives in
The matrix before this column is dealt with. The boxed entry is the pivot, the entry this step works with; the marked entries below it are the ones being driven to zero. Every other number is carried along unchanged by the moves.
3

Pivot 2: column 2, row 2

4

Read off the inverse

5

Prove it by multiplying back