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