a) \(\dfrac{x+y}{x+2y}:\dfrac{x+3y}{x+2y}:\dfrac{x+y}{x+3y}\)
\(=\dfrac{x+y}{x+2y}:\left(\dfrac{x+3y}{x+2y}\cdot\dfrac{x+y}{x+3y}\right)\)
\(=\dfrac{x+y}{x+2y}:\dfrac{x+y}{x+2y}\)
\(=1\)
b) \(\dfrac{x+y}{x+2y}:\left(\dfrac{x+3y}{x+2y}:\dfrac{x+y}{x+3y}\right)\)
\(=\dfrac{x+y}{x+2y}:\left(\dfrac{x+3y}{x+2y}\cdot\dfrac{x+3y}{x+y}\right)\)
\(=\dfrac{x+y}{x+2y}:\dfrac{\left(x+3y\right)^2}{\left(x+y\right)\left(x+2y\right)}\)
\(=\dfrac{x+y}{x+2y}\cdot\dfrac{\left(x+y\right)\left(x+2y\right)}{\left(x+3y\right)^2}\)
\(=\dfrac{\left(x+y\right)^2}{\left(x+3y\right)^2}\)