- #1
butterfli
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I finished this problem but I'm not sure if what I did was mathematically legal. This is a homework problem but I'm hesitant to turn it in the way it is since it's worth a lot of points. If someone can confirm what I did is correct or incorrect, I'd really appreciate it, thanks!
Prove that if a matrix A is idempotent (A^2 = A), then the determinant of A is 0 (det(A) = 0).
None
1) A^2 = A
2) AA = A
3) AA - A = A - A <- Here, I subtracted A from both sides
4) AA - A = 0 <- property of the Zero matrix (A+(-A) = 0)
5) A(I-A) = 0 <- I factored out A here, not sure if this is legal or not in matrices.
A(I-A) = 0 implies that either I-A = 0 or A = 0; det(0) = 0, therefore, det(A) = 0.
Homework Statement
Prove that if a matrix A is idempotent (A^2 = A), then the determinant of A is 0 (det(A) = 0).
Homework Equations
None
The Attempt at a Solution
1) A^2 = A
2) AA = A
3) AA - A = A - A <- Here, I subtracted A from both sides
4) AA - A = 0 <- property of the Zero matrix (A+(-A) = 0)
5) A(I-A) = 0 <- I factored out A here, not sure if this is legal or not in matrices.
A(I-A) = 0 implies that either I-A = 0 or A = 0; det(0) = 0, therefore, det(A) = 0.