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

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

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

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