A:CO2(g)+2NH3(g)CO(NH2)2(s) + H2O(g);△H<0
B:CO2(g)+ H2(g)CO(g) + H2O(g);△H>0
C:CH3CH2OH(g) CH2=CH2(g) + H2O(g);△H>0
D:2C6H5CH2CH3(g) + O2(g) 2C6H5CH=CH2(g) + 2 H2O(g);△H<0
已知|AB|=2,动点P满足|PA|=2|PB|,试建立恰当的直角坐标系,动点P的轨迹方程为____.