42  Pascal User Manual  var  SeriesLR,  {  series sum left  to  right in succession}  SumLRPos,  {  sum of  positive  terms,  left to  right  }  SumLRNeg,  {  sum of negative terms,  left  to  right  }  SeriesRL,  {  series sum right  to left  in  succession}  SumRLPos,  {  sum of positive terms,  right  to  left  }  SumRLNeg,  {  sum of  negative  terms,  right to left  }  PosTermLR,  {  next positive term,  left to right  }  NegTermLR,  {  next negative term,  left  to  right  }  PosTermRL,  {  next  positive term,  right to  left  }  NegTermRL: Real  {  next negative term right  to left  }  ;  PairsOfTerms:  Integer { count of pairs of terms };  begin  SeriesLR .- 0;  SumLRPos.- 0;  SumLRNeg.- 0;  SeriesRL .- 0;  SumRLPos·= 0;  SumRLNeg·= 0;  for PairsOfTerms  := 1 to 5000 do  begin  PosTermLR  .=  1  /  (2  *  PairsOfTerms  - 1);  NegTermLR  1  /  (2  PairsOfTerms);  PosTermRL  .- 1  /  (10001  - 2  *  PairsOfTerms);  NegTermRL  .=  1  /  (10002  - 2  PairsOfTerms);  SeriesLR  .=  SeriesLR  +  PosTermLR  SumLRPos  .- SumLRPos  +  PosTermLR;  SumLRNeg  SumLRNeg  +  NegTermLR;  SeriesRL  .- SeriesRL  +  PosTermRL  SumRLPos  SumRLPos  +  PosTermRL;  SumRLNeg  .- SumRLNeg  +  NegTermRL;  end;  Writeln(Output,  SeriesLR);  Writeln(Output,  SumLRPos - SumLRNeg);  Write In (Output,  SeriesRL);  Writeln(Output,  SumRLPos - SumRLNeg)  end .  - NegTermLR;  - NegTermRL;  Produces as results:  Why do the four "identical" sums differ?  6.930919E-01  6.931014E-01  6.930970E-01  6.930971E-01 