点P在X轴上,则P(X,0,0)
距离相等,距离的平方相等:
D^2=(X-1)^2+(0-2)^2+(0-1)^2=(X-2)^2+(0-2)^2+(0-2)^2
即(X-1)^2+5=(X-2)^2+8
X=3
P(3,0,0)