Thực hiện phép tính: a) (4x/(x + 7)).3(x + 7)/x b) 1/(x - 3) + 1/(x + 3)

Hướng dẫn giải

a) \[\frac{{4x}}{{\left( {x + 7} \right)y}} \cdot \frac{{3\left( {x + 7} \right)}}{x}\]

\[ = \frac{{4x \cdot 3\left( {x + 7} \right)}}{{\left( {x + 7} \right)y \cdot x}} = \frac{{12}}{y}.\]

b) \[\frac{1}{{x - 3}} + \frac{1}{{x + 3}}\]

\[ = \frac{{x + 3}}{{\left( {x - 3} \right)\left( {x + 3} \right)}} + \frac{{x - 3}}{{\left( {x - 3} \right)\left( {x + 3} \right)}}\]

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