Quy đồng mẫu thức các phân thức sau:

\(\frac{{4x - 4}}{{2x\left( {x + 3} \right)}}\) và \(\frac{{x - 3}}{{3x\left( {x + 1} \right)}}\).

Ta có \(\frac{{4x - 4}}{{2x\left( {x + 3} \right)}} = \frac{{2\left( {2x - 2} \right)}}{{2x\left( {x + 3} \right)}} = \frac{{2x - 2}}{{x\left( {x + 3} \right)}}\).

Mẫu thức chung: 3x(x + 3)(x + 1).

Ta có:

3x(x + 3)(x + 1) : x(x + 3) = 3(x + 1)

3x(x + 3)(x + 1) : 3x(x + 1) = (x + 3)

Quy đồng mẫu thức ta có:

\(\frac{{2x - 2}}{{x\left( {x + 3} \right)}} = \frac{{\left( {2x - 2} \right).3\left( {x + 1} \right)}}{{x\left( {x + 3} \right).3\left( {x + 1} \right)}} = \frac{{3\left( {2x - 2} \right)\left( {x + 1} \right)}}{{3x\left( {x + 3} \right)\left( {x + 1} \right)}}\)

\( = \frac{{6\left( {x - 1} \right)\left( {x + 1} \right)}}{{3x\left( {x + 3} \right)\left( {x + 1} \right)}} = \frac{{6\left( {{x^2} - 1} \right)}}{{3x\left( {x + 3} \right)\left( {x + 1} \right)}}\);

\[\frac{{x - 3}}{{3x\left( {x + 1} \right)}} = \frac{{\left( {x - 3} \right).\left( {x + 3} \right)}}{{3x\left( {x + 1} \right).\left( {x + 3} \right)}} = \frac{{\left( {x - 3} \right)\left( {x + 3} \right)}}{{3x\left( {x + 3} \right)\left( {x + 1} \right)}} = \frac{{{x^2} - 9}}{{3x\left( {x + 3} \right)\left( {x + 1} \right)}}\].