set A = { (((1 - lambda) * x1) + (lambda * x2)) where lambda is Real : verum } ;
{ (((1 - lambda) * x1) + (lambda * x2)) where lambda is Real : verum } c= REAL n
hence
{ (((1 - lambda) * x1) + (lambda * x2)) where lambda is Real : verum } is Subset of (REAL n)
; verum