reserve x, y for object, X for set,
  i, j, k, l, n, m for Nat,
  D for non empty set,
  K for commutative Ring,
  a,b for Element of K,
  perm, p, q for Element of Permutations(n),
  Perm,P for Permutation of Seg n,
  F for Function of Seg n,Seg n,
  perm2, p2, q2, pq2 for Element of Permutations(n+2),
  Perm2 for Permutation of Seg (n+2);
reserve s for Element of 2Set Seg (n+2);
reserve pD for FinSequence of D,
  M for Matrix of n,m,D,
  pK,qK for FinSequence of K,
  A for Matrix of n,K;

theorem Th51:
  for i, j st i in Seg n & j in Seg n & i <> j holds Det RLine(A,i
  ,Line(A,j)) = 0.K
proof
  let i, j such that
A1: i in Seg n and
A2: j in Seg n and
A3: i <> j;
A4: i<j or j<i by A3,XXREAL_0:1;
  len Line(A,j) = width A by MATRIX_0:def 7;
  then
A5: Line(RLine(A,i,Line(A,j)),i)=Line(A,j) by A1,Th28;
  Line(RLine(A,i,Line(A,j)),j)=Line(A,j) by A2,A3,Th28;
  hence thesis by A1,A2,A5,A4,Th50;
end;
