Consider the following example:

1 2 3

4 5 6

7 8 9

The Sum of principal diagonal elements = 1 + 5 + 9 = 15

The Sum of secondary diagonal elements = 3 + 5 + 7 =15

Consider a square matrix of size 3

A[0,0] A[0,1] A[0,2]

A[1,0] A[1,1] A[1,2]

A[2,0] A[2,1] A[2,2]

Sum of principal diagonal elements are = A[0,0] + A[1,1] + A[2,2]

Su of secondary diagonal elements are = A[0,2] + A[1,1] + A[2,0]