MATLAB: How to find a tangent point from a circle

tangent point

I have a code to find a tangent point from a circle.I have used the circle radius as 2000.For all the points on the circle it will find the tangential point for the given radius. But when the point lies exactly on the same line then the vector point will be NAN for the given radius,if the radius changed then the vector is having answer why?Here i have displayed code for finding points.kindly give answer.
x1=200000;y1=15000;
x2=350000;y2=20000;
z1=200;z2=200;
x3,y3,z3=[230000 16000 200];
Radius=2000;
a11=((x2-x1)^2+(y2-y1)^2+(z2-z1)^2);
b11=2*(((x2-x1)*(x1-x3))+((y2-y1)*(y1-y3))+((z2-z1)*(z1-z3)));
c11=x3.^2+y3.^2+z3.^2+(x1^2)+(y1^2)+(z1^2)-2*((x3*x1)+(y3*y1)+(z3*z1))-(Radius*Radius);
condn1=(b11*b11)-(4*a11*c11);
condition=abs(condn1);
t=(-b11+(sqrt((condition))))/(2*a11);
t1=(-b11-(sqrt((condition))))/(2*a11);
Xi=x1+(x2-x1)*t;
Yi=y1+(y2-y1)*t;
Zi=z1+(z2-z1)*t;                      
Xf=x1+(x2-x1)*t1;
Yf=y1+(y2-y1)*t1;
Zf=z1+(z2-z1)*t1;                     
v1 = [Xi-x3;Yi-y3;Zi-z3];
radius = norm(v1);
v2 = [Xf-x3;Yf-y3;Zf-z3];
v3 = cross(cross(v1,v2),v1);
v3 = radius*v3/norm(v3);
t = linspace(atan2(norm(cross(v1,v2)),dot(v1,v2)),0,5);
vec = v1*cos(t)+v3*sin(t);
XVec=vec(1,:)+x3
YVec=vec(2,:)+y3
ZVec=vec(3,:)+z3
Related Question