In this segment, we'll talk about
how to find a trace of a matrix.
And how do we define the trace of a matrix? First it will be
matrix has
to be square. So the trace of a 
square matrix [A],
so it has n rows and n columns, is defined as
--we'll call the trace as "tr"--
so tr[A] is summation 
i is equal to 1 to n
aii. So all you are doing in order to find the trace of a 
matrix is simply take all the
diagonal elements and adding them all up.
So that's what the trace of a square matrix is.
Let's take an example. 
Let's suppose somebody says hey [A] matrix is given as
[15 6 7, 3 9 2, -1 3 -7].
So if that's the
[A] matrix and somebody says hey find out the trace of this
matrix. I'll say trace of [A] is summation I is equal to 
1 to 3 aii. Because we have three
the size of this matrix is three by three. So that will become
a11 plus a22 plus a33.
So it's basically the summation of all the 
diagonal elements, but I'm expanding it just
to show you how the formula works. 
Or if you were going to program this, how
you should interpret that.
So that becomes 15+9-7=17
So that's what the trace of this particular
matrix is.
And that's the end of this segment.
