Найдите значение выражения: ((a+7b)/(a^2-7ab)-(a-7b)/(a^2+7ab))*(49b^2-a^2)/7b^2 при a=-2/3; b=4 3/8.

\[\left( \frac{a + 7b}{a^{2} - 7ab} - \frac{a - 7b}{a^{2} + 7ab} \right) \cdot \frac{49b^{2} - a^{2}}{7b^{2}} =\]

\[= \frac{28\text{ab}}{a\left( a^{2} - 49b \right)} \cdot \frac{49b^{2} - a^{2}}{7b^{2}} = - \frac{4}{b}.\]

\[b = 4\frac{3}{8} = \frac{35}{8}:\]

\[- \frac{4}{b} = - 4\ :\frac{35}{8} = - 4 \cdot \frac{8}{35} = - \frac{32}{35}.\]