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$1.$
$a) frac{15xy^4}{10x^3y^2} = frac{3y^2}{2x^2}$

$b) frac{ab-b}{b^2}= frac{b(a-1)}{b^2}= frac{a-1}{b}$

$B) frac{4x^2-y^2}{2x-y} = frac{(2x-y)(2x+y)}{(2x-y)}= frac{2x+y}{1} = 2x+y$
$2.$

$a) frac{3}{a}+ frac{a-3}{a+5} = frac{3a+15+a^2-3a}{a(a+5)} = frac{a^2+15}{a(a+5)}$

$b) frac{2x^2}{x^2-4} - frac{2x}{x+2}= frac{2x^2}{(x-2)(x+2)} - frac{2x(x-2)}{(x-2)(x+2)} = frac{2x^2-2x^2+4x}{x^2-4}=frac{4x}{(x-2)(x+2)}$

$B) frac{7a}{a-b}-7 = frac{7a-7a+7b}{a-b} = frac{7b}{a-b}$
$3.$
$frac{5}{(a+2)^2}-frac{5}{a^2-4}-frac{5}{a+2}= frac{5}{(a+2)(a+2)} - frac{5}{(a-2)(a+2)} - frac{5}{a+2}=$ $frac{(5a-10)-(5a+10)-(5(a^2-4))}{(a+2)(a+2)(a-2)} = frac{-20-5a^2+20}{(a+2)(a+2)(a-2)} = - frac{5a^2}{(a+2)(a+2)(a-2)}$
$4.$
   $frac{2a-2c+ax-cx}{x^2-4} = frac{2(a-c)+x(a-c)}{(x+2)(x-2)} = frac{(2+x)(a-c)}{(x+2)(x-2)}$
если, $a=6.7;c=5.3;x=1.9$ , то $frac{(2+1.9)(6.7-5.3)}{(1.9+2)(1.9-2)} = frac{3.9*1.4}{3.9*(-0.1)} = frac{1.4}{-0,1} =- frac{14}{-1} =-14$
$5.$
   $frac{(x+2)^2}{2} - frac{x^2-4}{4}- frac{(x-2)^2}{8} = frac{x^2}{8}$    $|*8$
   $4(x+2)(x+2)-2(x+2)(x-2)-(x-2)(x-2)=x^2$
   $20x=-20$
   $x=-1$
Ответ: -1