Task

Find and display the sum of elements that are below the main diagonal of a matrix.

The matrix should be a square matrix.



 ───   Matrix to be used:   ───


     [[1,3,7,8,10],
      [2,4,16,14,4],
      [3,1,9,18,11],
      [12,14,17,18,20],
      [7,1,3,9,5]] 



using LinearAlgebra
 
A = [ 1  3  7  8 10;
      2  4 16 14  4;
      3  1  9 18 11;
     12 14 17 18 20;
      7  1  3  9  5 ]
 
@show tril(A)
 
@show tril(A, -1)
 
@show sum(tril(A, -1))  # 69
 