Tính:

\(\frac{{1 - 4{x^2}}}{{{x^2} + 4x}}:\frac{{2 - 4x}}{{3x}}\).

\(\frac{{1 - 4{x^2}}}{{{x^2} + 4x}}:\frac{{2 - 4x}}{{3x}} = \frac{{\left( {1 - 2x} \right)\left( {1 + 2x} \right)}}{{x\left( {x + 4} \right)}}:\frac{{2\left( {1 - 2x} \right)}}{{3x}}\)

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

\( = \frac{{\left( {x - y} \right)\left( {x + y} \right).3xy}}{{6{x^2}y.\left( {x + y} \right)}} = \frac{{x - y}}{{2x}}\).