Solve the following system of equations by matrix method:

6x – 12y + 25z = 4


4x + 15y – 20z = 3


2x + 18y + 15z = 10

The given system can be written in matrix form as:


or A X = B


A = , X = and B =


Now, |A| = 6


= 6(225 + 360) + 12(60 + 40) + 25(72 – 30)


= 3510 + 1200 + 1050


= 5760


So, the above system has a unique solution, given by


X = A – 1B


Cofactors of A are:


C11 = (– 1)1 + 1 (225 + 360) = 585


C21 = (– 1)2 + 1 (– 180 – 450) = 630


C31 = (– 1)3 + 1 (240 – 375) = – 135


C12 = (– 1)1 + 2 (60 + 40) = – 100


C22 = (– 1)2 + 1 (90 – 50) = 40


C32 = (– 1)3 + 1 (– 120 – 100) = 220


C13 = (– 1)1 + 2 (72 – 30) = 42


C23 = (– 1)2 + 1(108 + 24) = – 132


C33 = (– 1)3 + 1 (90 + 48) = 138


adj A =


=


Now, X = A – 1B =


X =


X =


Hence, X = ,Y = and Z =


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