$\frac{a^2 - 81}{a^2 - 8a} : \frac{a - 9}{a^2 - 64} = \frac{(a - 9)(a + 9)}{a(a - 8)} : \frac{a - 9}{(a - 8)(a + 8)} = \frac{(a - 9)(a + 9)}{a(a - 8)} * \frac{(a - 8)(a + 8)}{a - 9} = \frac{a + 9}{a} * \frac{a + 8}{1} = \frac{(a + 9)(a + 8)}{a}$
при a = −4:
$\frac{(a + 9)(a + 8)}{a} = \frac{(-4 + 9)(-4 + 8)}{-4} = \frac{5 * 4}{-4} = -5$

$\frac{x}{4x^2 - 4y^2} : \frac{1}{6x + 6y} = \frac{x}{(2x - 2y)(2x + 2y)} : \frac{1}{3(2x + 2y)} = \frac{x}{(2x - 2y)(2x + 2y)} * \frac{3(2x + 2y)}{1} = \frac{x}{2x - 2y} * \frac{3}{1} = \frac{3x}{2(x - y)}$
при x = 4,2, y = −2,8:
$\frac{3x}{2(x - y)} = \frac{3 * 4,2}{2(4,2 - (-2,8))} = \frac{12,6}{2(4,2 + 2,8)} = \frac{12,6}{2 * 7} = \frac{12,6}{14} = \frac{126}{140} = \frac{9}{10} = 0,9$

$(3a^2 - 18a + 27) : \frac{3a - 9}{4a} = 3(a^2 - 6a + 9) : \frac{3(a - 3)}{4a} = 3(a - 3)^2 * \frac{4a}{3(a - 3)} = (a - 3) * \frac{4a}{1} = 4a(a - 3)$
при a = 0,5:
4a(a − 3) = 4 * 0,5(0,5 − 3) = 2 * (−2,5) = −5

$\frac{a^6 + a^5}{(3a - 3)^2} : \frac{a^5 + a^4}{9a^2 - 9a} = \frac{a^5(a + 1)}{9(a - 1)^2} : \frac{a^4(a + 1)}{9a(a - 1)} = \frac{a^5(a + 1)}{9(a - 1)^2} * \frac{9a(a - 1)}{a^4(a + 1)} = \frac{a}{a - 1} * \frac{a}{1} = \frac{a^2}{a - 1}$
при a = 0,8:
$\frac{a^2}{a - 1} = \frac{0,8^2}{0,8 - 1} = \frac{0,64}{-0,2} = -\frac{64}{20} = -\frac{32}{10} = -3,2$