theorem Th49:
  ZeroPointSet( LF(a2,b2,c2))<>{} implies
    ex x,y be Element of INT st
      |.LF(a1,b1,c1).(x,y).|*|.LF(a2,b2,c2).(x,y).|<=|.a1*b2-a2*b1.|/4
  proof
    assume ZeroPointSet( LF(a2,b2,c2))<>{}; then
    consider p be object such that
A2: p in ZeroPointSet(LF(a2,b2,c2)) by XBOOLE_0:def 1;
    LF(a2,b2,c2).p = 0 & p in dom LF(a2,b2,c2) by A2,Th13; then
    consider p1,p2 be object such that
A4: p1 in INT & p2 in INT & p = [p1,p2] by ZFMISC_1:def 2;
    reconsider x = p1,y=p2 as Element of INT by A4;
    LF(a2,b2,c2).(x,y)=0 by A2,Th13,A4; then
    |.LF(a1,b1,c1).(x,y).|*|.LF(a2,b2,c2).(x,y).|
    <= |.a1*b2-a2*b1.|/4;
    hence thesis;
  end;
