# Math Help - inverses of matrices

1. ## inverses of matrices

I know that:
$\begin{bmatrix} a & b \\ c & d \end{bmatrix}^{-1}=\frac{1}{Determinant} \begin{bmatrix} d & -b \\ -c & a \end{bmatrix}$

But what is the equivalent expression for 3x3 (or higher dimensional matrices)?

Thanks

2. Originally Posted by DivideBy0
I know that:
$\begin{bmatrix} a & b \\ c & d \end{bmatrix}^{-1}=\frac{1}{Determinant} \begin{bmatrix} d & -b \\ -c & d \end{bmatrix}$

But what is the equivalent expression for 3x3 (or higher dimensional matrices)?

Thanks
The best way to find inverses is by Gaussian-Jordan elimination. Because that is the most efficient algorithm. However, it is not pleasant. There is a very elegant way to find inverses but when matrices get large it because computationally difficult to do (still it has important theoretical uses).

Step 1: Given $A$ a $n\times n$ matrix compute $\det A$. If this determinant is zero then it have no inverse.

Step 2: Compute the cofactor matrix. Meaning for each $a_{ij}$ entry in the matrix compute the cofactor of $a_{ij}$. Replace each entry by its cofactor. This will form a matrix called the cofactor matrix.

Step 3: Compute the adjoint matrix. To do this find the transpose of the cofactor matrix. The transpose operation on $M$ (denoted by $M^T$) is flipping the elements along the main diagnol. So for the $a_{ij}$ entry replace by $a_{ji}$ entry. (Note, that the main diagnol of the matrix is unchanged by transpose operation).

Step 4: Now divide each element in the adjoint matrix by the determinant. The resulting matrix is the inverse.

3. Wow thanks... that is a lot to do to simply find an inverse...

Maybe that's why my book didn't explain how to do it.

Does anyone know how to find the inverse on a calculator?

4. Originally Posted by DivideBy0
Wow thanks... that is a lot to do to simply find an inverse...

Maybe that's why my book didn't explain how to do it.

Does anyone know how to find the inverse on a calculator?
I do not own any of those new fancy show-off calculators. But I remeber from playing around with one of the TI's that I found a way to find inverse of matrices. There is a matrix command bottom. Program your matrix then press in the matrix into the screen followed by $\boxed{^{-1}}$ and then enter. This will give you a matrix which is the inverse.