设y1=k1x² y2=k2(x-2)
∴y=k1x²+k2(x-2)
x=1时y=0
∴k1-k2=0
x=3时 y=4
∴9k1+k2=4
∴k1=2/5
k2=2/5
∴y=2x²/5+2(x-2)/5