$(\frac{a^2 - 5a}{a^2 - 10a + 25} + \frac{25}{a^2 - 25}) : \frac{125 - a^3}{5 + a} = (\frac{a(a - 5)}{(a - 5)^2} + \frac{25}{(a - 5)(a + 5)}) : \frac{125 - a^3}{a + 5} = (\frac{a}{a - 5} + \frac{25}{(a - 5)(a + 5)}) * \frac{a + 5}{125 - a^3} = \frac{a(a + 5) + 25}{(a - 5)(a + 5)} * \frac{a + 5}{125 - a^3} = \frac{a^2 + 5a + 25}{a - 5} * \frac{1}{(5 - a)(25 + 5a + a^2)} = \frac{1}{a - 5} * \frac{1}{5 - a} = \frac{1}{a - 5} * (-\frac{1}{a - 5}) = -\frac{1}{(a - 5)^2}$
при a = 4,5:
$-\frac{1}{(4,5 - 5)^2} = -\frac{1}{(-0,5)^2} = -\frac{1}{(\frac{1}{2})^2} = -\frac{1}{\frac{1}{4}} = -4$