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: 

6.930919E-01 6.931014E-01 6.930970E-01 6.930971E-01 

Why do the four "identical" sums differ? 

