You cannot, unless you also change the value of the fraction.
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First, change it so that the two fractions have the same denominator (by changing the fractions into equivalent fractions). Once the two fractions have the same denominator, it is simply a case of subtracting the numerators, leaving the denominator the same. Finally, reduce the fraction to its lowest terms (if possible).
it means when two denominators can reduce fast
You should reduce a fraction if the numerator and denominator have any common factor greater than one.
You divide both the numerator and denominator by a common multiple.
Multiply the denominator by the whole number. Add the numerator to that total. Put that sum over the original denominator.