--- Lib/fractions.py	2014-08-22 18:59:30.623831217 +0200
+++ Lib/fractions.py	2014-09-22 23:04:26.748545375 +0200
@@ -399,17 +407,17 @@
 
     def _add(a, b):
         """a + b"""
-        return Fraction(a.numerator * b.denominator +
-                        b.numerator * a.denominator,
-                        a.denominator * b.denominator)
+        da, db = a.denominator, b.denominator
+        return Fraction(a.numerator * db + b.numerator * da,
+                        da * db)
 
     __add__, __radd__ = _operator_fallbacks(_add, operator.add)
 
     def _sub(a, b):
         """a - b"""
-        return Fraction(a.numerator * b.denominator -
-                        b.numerator * a.denominator,
-                        a.denominator * b.denominator)
+        da, db = a.denominator, b.denominator
+        return Fraction(a.numerator * db - b.numerator * da,
+                        da * db)
 
     __sub__, __rsub__ = _operator_fallbacks(_sub, operator.sub)