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

Так как \(1-y=-(y-1)\), получаем:

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

Ответ: \(\frac{1-y}{1+y}\)