Giải phương trình:
\(\frac{x-4}{x\left(x+2\right)}\) - \(\frac{1}{x\left(x-2\right)}\) = \(\frac{-2}{\left(x+2\right)\left(x-2\right)}\)
\(\frac{1}{x\left(x+3\right)}\) + \(\frac{1}{\left(x+3\right)\left(x+6\right)}\) + \(\frac{1}{\left(x+6\right)\left(x+9\right)}\) + \(\frac{1}{\left(x+9\right)\left(x+12\right)}\) = \(\frac{1}{16}\)