\[\frac{2a + b}{a - 2b} + \frac{a + 3b}{2b - a} =\]
\[= \frac{2a + b}{a - 2b} - \frac{a + 3b}{a - 2b} =\]
\[= \frac{2a + b - a - 3b}{a - 2b} = \frac{a - 2b}{a - 2b} = 1.\]