10 y-9 y²-81 y-8 y²-16y+6+

10) $$ \frac{y-9}{y-8} : \frac{y^2-81}{y^2-16y+64} = \frac{y-9}{y-8} : \frac{(y-9)(y+9)}{(y-8)^2} = \frac{y-9}{y-8} \cdot \frac{(y-8)^2}{(y-9)(y+9)} = \frac{(y-9)(y-8)(y-8)}{(y-8)(y-9)(y+9)} = \frac{y-8}{y+9} $$

Ответ: $$\frac{y-8}{y+9}$$