法向量n1=(2x0,2y0,-1),n2=(1,0,2),n3=(0,1,2),满足n1×（n2×n3)=0,求得在x0,y0处n1=(2,2,-1)所以所求点为(1,1,2),n=(2,2-1),切平面为2(x-1)+2(y-1)-(z-2)=0