reserve a,b,n for Element of NAT;

theorem Th5:
  (a+b) |^ 2 = a*a + a*b + a*b + b*b
proof
  (a+b)|^2=(a+b)*(a+b) by WSIERP_1:1
    .=a*a+a*b+b*a+b*b;
  hence thesis;
end;
