焦点在x轴上的椭圆x^2/4+y^2/b^2=1(b>0),F1,F2是两焦点,椭圆上存在P点使得向量PF1•向量PF2=0,求b的取值范围.设P(x,y),向量PF1•向量PF2=0,则PF1⊥PF2,y/(x+c)•y/(x-c)=-1,x²+y²=c²,...