例1 (2017浙江杭州质检,19)在平面直角坐标系内,点A(0,1),B(0,-1),C(1,0),点P满足 · =k| |2. (1)若k=2,求点P的轨迹方程; (2)当k=0时,若|λ + |max=4,求实数λ的值.