Tính tổng sau:
a, \(\dfrac{-1}{x^2-x}+\dfrac{-1}{x^2-3x+2}+\dfrac{-1}{x^2-5x+6}+\dfrac{-1}{x^2-7x+12}+\dfrac{-1}{x^2-9x+20}+\dfrac{1}{x-5}\)
b, \(\dfrac{3}{x\left(x+3\right)}+\dfrac{3}{\left(x+3\right)\left(x+6\right)}+\dfrac{3}{\left(x+6\right)\left(x+9\right)}+\dfrac{1}{x+9}\)