SOLUTION: A matrix is given- [ 2k-1 3 3 3 2k-1 3 3 3 2k-1 ] Find value of k when matrix is singular.

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Question 934861: A matrix is given-
[ 2k-1 3 3
3 2k-1 3
3 3 2k-1 ]
Find value of k when matrix is singular.

Answer by MathLover1(20850) About Me  (Show Source):
You can put this solution on YOUR website!
A matrix is given-

%28matrix%283%2C3%2C2k-1%2C3%2C3%2C3%2C2k-1%2C3%2C3%2C3%2C2k-1%29%29
Find value of k when matrix is singular.

A matrix is non-invertable, or singular, when its determinant is zero; so, find its determinant in terms of k, then set that to zero and solve for k.
here, determinant is D=3%2B3%2B2k-1, then we have
3%2B3%2B2k-1=0....solve for k
6%2B2k-1=0
5%2B2k=0
2k=-5
highlight%28k=-5%2F2%29 => then we have 2k-1=2%28-5%2F2%29-1=-5-1=-6
plug this value in given matrix
%28matrix%283%2C3%2C-6%2C3%2C3%2C3%2C-6%2C3%2C3%2C3%2C-6%29%29
check if determinant is equal to zero

Solved by pluggable solver: Finding the Determinant of a 3x3 Matrix

If you have the general 3x3 matrix:

%28matrix%283%2C3%2Ca%2Cb%2Cc%2Cd%2Ce%2Cf%2Cg%2Ch%2Ci%29%29

the determinant is:

Which further breaks down to:



Note: abs%28matrix%282%2C2%2Ce%2Cf%2Ch%2Ci%29%29, abs%28matrix%282%2C2%2Cd%2Cf%2Cg%2Ci%29%29 and abs%28matrix%282%2C2%2Cd%2Ce%2Cg%2Ch%29%29 are determinants themselves.
If you need help finding the determinant of 2x2 matrices (which is required to find the determinant of 3x3 matrices), check out this solver

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From the matrix %28matrix%283%2C3%2C-6%2C3%2C3%2C3%2C-6%2C3%2C3%2C3%2C-6%29%29, we can see that a=-6, b=3, c=3, d=3, e=-6, f=3, g=3, h=3, and i=-6

Start with the general 3x3 determinant.

Plug in the given values (see above)

Multiply

Subtract

abs%28matrix%283%2C3%2C-6%2C3%2C3%2C3%2C-6%2C3%2C3%2C3%2C-6%29%29=-162--81%2B81 Multiply

abs%28matrix%283%2C3%2C-6%2C3%2C3%2C3%2C-6%2C3%2C3%2C3%2C-6%29%29=0 Combine like terms.


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Answer:

So abs%28matrix%283%2C3%2C-6%2C3%2C3%2C3%2C-6%2C3%2C3%2C3%2C-6%29%29=0, which means that the determinant of the matrix %28matrix%283%2C3%2C-6%2C3%2C3%2C3%2C-6%2C3%2C3%2C3%2C-6%29%29 is 0