1)2)При а=25,в=56

1)
$\frac{y^2}{x-xy} : (\frac{y}{x-y}- \frac{2xy}{x^2-y^2})= \frac{y^2}{x(1-y)} : (\frac{y(x+y)}{(x-y)(x+y)}- \frac{2xy}{(x-y)(x+y)})= \\ = \frac{y^2}{x(1-y)} : \frac{yx+y^2-2xy}{(x-y)(x+y)}}=\frac{y^2}{x(1-y)} : \frac{y^2-xy}{(x-y)(x+y)}}= \\ \frac{y^2}{x(1-y)} : \frac{y(y-x)}{(x-y)(x+y)}}= \frac{y^2}{x(1-y)} * \frac{(x-y)(x+y)}{y(y-x)}}=\\ = \frac{y^2}{x(1-y)} * \frac{(x-y)(x+y)}{-y(x-y)}}=\frac{y^2}{x(1-y)} * \frac{(x+y)}{-y}}= \\ =\frac{y}{-x(y-1)} * \frac{(x+y)}{-1}}= \frac{y(x+y)}{x(1-y)} =$
$= \frac{yx+y^2}{x-xy}$

2) Мне кажется в первом числителе 25a. а не 25n
$\frac{25a^2-b^2}{25a^2-10ab+b^2}:(5a^2+ab)*(2b-10a)= \\ \frac{(5a)^2-b^2}{(5a)^2-2*5ab+b^2}* \frac{1}{a(5a+b)}*2(b-5a)= \\ \frac{(5a+b)(5a-b)}{(5a-b)^2}* \frac{1}{a(5a+b)}*(-2)(5a-b)= \\ \frac{(5a+b)(5a-b)}{(5a-b)}* \frac{1}{a(5a+b)}*(-2)=\frac{(5a+b)(5a-b)}{(5a-b)}* \frac{1}{a(5a+b)}*(-2)= \\ =\frac{(5a-b)}{(5a-b)}* \frac{1}{a}*(-2)=\frac{1}{1}* \frac{1}{a}*(-2)=-2/a$