设动弦中点坐标是A(x,y)
则A与圆心O(0,0)连线AO必垂直于AP
AP的斜率是：(y-2)/(x-1)
AO的斜率是：(y-0)/(x-0)
所以：[(y-2)/(x-1)][(y-0)/(x-0)]=-1
y(y-2)+x(x-1)=0
x²+y²-2y-x=0,此即动弦中点的轨迹方程