clear;clc;
a=2,b=3,c=4,d=5,
p=[1 2 3],
hold on;
[x,y,z]=meshgrid(linspace(-5,5,25));
isosurface(x,y,z,a*x+b*y+c*z+d,0);
t=[0,(a*p(1)+b*p(2)+c*p(3)+d)/(a^2+b^2+c^2)];
plot3(p(1)-a*t,p(2)-b*t,p(3)-c*t,'.-');
view(3);axis equal;grid on;
text(p(1),p(2),p(3),' P ');