Matrices math question homework

1. Jun 3, 2006

wayneo

determine the value of k for which the matrix

(2 4 1)
(k -1 2)
(7 7 4) has no inverse

apparantly the answer is k = 1 but I cant see how.

I worked it out to be -3

2. Jun 3, 2006

dav2008

I obtained an answer that isn't k=1, by the way. Are you sure you copied the matrix and/or the answer down correctly ?

Last edited: Jun 3, 2006
3. Jun 3, 2006

finchie_88

I agree that it is not one, but I don't agree with your answer of -3. Assuming that the matrix you gave is correct, look back at your working (preferably post what you have done) - I checked my result with my calculator, and found the matrix to be singular.

4. Jun 4, 2006

wayneo

see I just wanted to be sure whether the book was wrong or not as it says it is 1. I tried working out the determinant step by step and making it equal to 0 then solve for 'k' like an algebra equation

5. Jun 4, 2006

jbusc

I worked it out to be neither 1 nor -3, and double checked in matlab.

rework and check your signs carefully.