15) \(\dfrac{(y-1)(y-1)}{(1-y)(1+y)}=\)

ОДЗ: \(y\ne 1\), \(y\ne -1\).

Так как \(1-y=-(y-1)\), получаем:

\[\frac{(y-1)(y-1)}{(1-y)(1+y)}=\frac{(y-1)^2}{-(y-1)(y+1)}=-\frac{y-1}{y+1}=\frac{1-y}{y+1}.\]

Ответ: \(\dfrac{1-y}{y+1}\), \(y\ne 1,-1\).