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# P6 matrices question watch

1. how do you find the inverse of a matrix such as

2 3 4
-1 9 11
7 -1 2

??
2. (Original post by lesser weevil)
how do you find the inverse of a matrix such as

2 3 4
-1 9 11
7 -1 2

??
(1) Find the determinant of the 3x3 matrix.
(2) Form the matrix of minors, by replacing each element of the 3x3 matrix with its minor. Note that the minor is equal to the determinant of the 2x2 matrix formed when you cover the row and column which the particular element lies in.
(3) Apply the law of alternating signs:
+ - +
- + -
+ - +
shows which minor signs you should change.
This produces the matrix of cofactors.
(4) Transpose the matrix of cofactors.
(5) Divide by the determinant, this gives the inverse.
3. Thanks!

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