# trace of matrix

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if $\lambda$3 - 6$\lambda$2 -$\lambda$ +22=0 is a characteristic of 3 X 3 diagonal matrix , then trace of matrix A is

The following points to be noted :

a) In a diagonal matrix D , determinant(D)  =  a.b.c.... where each of a,b, c and so on are diagonal elements.

b) The diagonal elements of  the diagonal matrix are eigen values as well..

c) Trace is the sum of the diagonal elements of a matrix..

d) The sum of eigen values is given by trace of the matrix.

e) The roots of characteristic equation are nothing but eigen values of the corresponding matrix

So , trace of a matrix is nothing but the sum of eigen values..

And hence given a characteristic equation , sum of eigen values is the same as sum of roots of the corresponding characteristic equation..

Given characteristic equation :    λ3 - 6λ2 - λ + 22 = 0

We know sum of roots here   =  - (Coefficient of λ) / (Coefficient of  λ)

=  6

Hence trace of matrix           =  6

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( t1 – $\lambda$ ) ( t2 – $\lambda$ ) ( t3 – $\lambda$ ) =0 is the characteristic equation. In this the coeffecient of $-\lambda ^{2}$ gives the trace.

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