\(\mathop {\lim
}\limits_{x \to 3}\frac {x^ {2 }-5x+6 } {x^ {2 }-8x+15 }= \mathop {\lim }\limits_{x
\to 3}\frac {(x-2)(x-3) } {(x-3)(x-5) }= \mathop {\lim }\limits_{x \to 3}\frac
{x-2 } {x-5 }=-\frac {1 } { 2 }\)
\( \mathop {\lim
}\limits_{x \to -1}\frac {x^ {3 }-2x-1 } {x^ {5 }-2x-1 }= \mathop {\lim
}\limits_{x \to -1}\frac {(x+1)(x^ {2 }-x-1) } {(x+1)(x^ {4 }-x^ {3 }+x^ {2
}-x-1) }\)
\(= \mathop {\lim
}\limits_{x \to -1} \frac {x^ {2 }-x-1 } {x^ {4 }-x^ {3 }+x^ {2 }-x-1 }=\frac
{1 } {3 }\)