
theorem
  for L being Field
  for a,c being Element of L
  for b,d being non zero Element of L
  holds SumRoots(<%a,b%>*'<%c,d%>) = (-a/b) + (-c/d)
  proof
    let L be Field;
    let a,c be Element of L;
    let b,d be non zero Element of L;
    SumRoots(<%a,b%>) = -a/b & SumRoots(<%c,d%>) = -c/d by Th27;
    hence thesis by Th28;
  end;
