11. (u !! v) is orthogonal or perpendicular to the plane determined by vectors u and v.
12. Given the matrix A = [1 -2; 1 4], the vector x = [1; 1] is an eigenvector.
13. The points with coordinates A(0,0), B(1,2) and C(3,6) are collinear.
14. The distance between the vectors v1 = [1; -5] and v2 = [-3; -2] is 5.
15. The sum of the diagonal elements is equal to the sum of the eigenvalues of a square matrix.