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]] 



SumBelowDiagonal ← +´∘⥊⊢×(>⌜´)∘(↕¨≢)
 
matrix ← >⟨⟨ 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⟩⟩
 
SumBelowDiagonal matrix