# Math Help - How to find inverse of matrix?

1. ## How to find inverse of matrix?

Hi

Is there any special setting that I need to have on since I am getting the wrong inverse for one of my questions.

This is my original martix

17 17 5
21 18 21
2 2 19

this should be the inverse

4 9 15
15 17 6
24 0 17

I am getting on my calc(stopped after 3rd decimal place but there is like 10 or more decimal places)

-.319 .333 -.284
.380 -.333 .268
-.006 1.249 .054

So that is totally different

I put in my matrix by going 2nd-matrix-edit

then to inverse I did [A] then x-1 to get the -1 on top of it.

I have the ti-83 plus calc.

2. Originally Posted by chobo2
Hi

Is there any special setting that I need to have on since I am getting the wrong inverse for one of my questions.

This is my original martix

17 17 5
21 18 21
2 2 19

Mr F says: Call it A.

this should be the inverse

4 9 15
15 17 6
24 0 17

Mr F says: Call it B.

I am getting on my calc(stopped after 3rd decimal place but there is like 10 or more decimal places)

-.319 .333 -.284
.380 -.333 .268
-.006 1.249 .054

So that is totally different

I put in my matrix by going 2nd-matrix-edit

then to inverse I did [A] then x-1 to get the -1 on top of it.

I have the ti-83 plus calc.
It's simple to show that AB does NOT equal I. B cannot possibly be the inverse of A.

3. Originally Posted by chobo2
Hi

Is there any special setting that I need to have on since I am getting the wrong inverse for one of my questions.

This is my original martix

17 17 5
21 18 21
2 2 19

this should be the inverse

4 9 15
15 17 6
24 0 17

I am getting on my calc(stopped after 3rd decimal place but there is like 10 or more decimal places)

-.319 .333 -.284
.380 -.333 .268
-.006 1.249 .054

So that is totally different

I put in my matrix by going 2nd-matrix-edit

then to inverse I did [A] then x-1 to get the -1 on top of it.

I have the ti-83 plus calc.
1. Your inverse matrix can't be right.

2. The calculator gave the correct matrix.

3. Use the MATH FRAC command to get the values as fractions.

4. The value 1.24... actually is $1.249201278 \cdot 10^{-15}$ that means nearly zero.

4. Originally Posted by earboth
1. Your inverse matrix can't be right.

2. The calculator gave the correct matrix.

3. Use the MATH FRAC command to get the values as fractions.

4. The value 1.24... actually is $1.249201278 \cdot 10^{-15}$ that means nearly zero.
This is part of a cipher called the "hill cipher" it uses the matrix math to do the stuff could it be that it does the inverse differently?

5. Originally Posted by chobo2
This is part of a cipher called the "hill cipher" it uses the matrix math to do the stuff could it be that it does the inverse differently?
You find your example here: Computer Musings: Hill Cipher Deciphered