解???$： (1)$???設(shè)???$y= ax2+ bx+2，$???把???$(1，$??????$1)(3，$??????$5)$???代入
得???$：\begin{cases}{a+b+2=1}\\{9a+3b+2=5} \end{cases}$???
解得???$a=1，b=-2$???
所以???$y= x2- 2x+2$???
???$(2)$???設(shè)???$y=a(x+1)2+2，$???
把???$(2，$??????$1)$???代入得???$9a+2= 1，$???
得???$a=-\frac {1}{9}$???
所以???$y=-\frac {1}{9}x2-\frac {2}{9}x+\frac {17}{9}$???
???$(3)$???設(shè)???$y=a(x+ 1)(x- 2)，$???
把???$(1 ， 2)$???代入得???$-2a=2，$???得???$a=-1$???
所以???$y= -(x+1)(x-2)= -x2+x+2$??
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