[tex] \frac{b}{2 a^{2}-ab} - \frac{4a}{2ab- b^{2}} = \\ \\ =\frac{b}{a(2a-b)} - \frac{4a}{b(2a- b)}= \\ \\ =\frac{b^{2}}{ab(2a-b)} - \frac{4a^{2}}{ab(2a- b)}= \\ \\=\frac{b^{2}-4a^{2}}{ab(2a-b)} = \frac{(b-2a)(b+2a)}{ab(2a-b)}= -\frac{(2a-b)(2a+b)}{ab(2a-b)}= -\frac{2a+b}{ab}[/tex]
