reserve G for Group;
reserve A,B for non empty Subset of G;
reserve N,H,H1,H2 for Subgroup of G;
reserve x,a,b for Element of G;

theorem
  for N,N1,N2 be Subgroup of G st N = N1 /\ N2 holds N1 ` A /\ N2 ` A c= N ` A
proof
  let N,N1,N2 be Subgroup of G;
  assume N = N1 /\ N2; then
A1:N is Subgroup of N1 & N is Subgroup of N2 by GROUP_2:88;
  let x be object;
  assume x in N1 ` A /\ N2 ` A; then
A2: x in N1 ` A & x in N2 ` A by XBOOLE_0:def 4;
  N1 ` A c= N ` A & N2 ` A c= N ` A by A1,Th25;
  hence thesis by A2;
end;
