MATLAB: How to draw a tangent line from a given point to a circle in Matlab

Question

Hi I am trying to draw a tangent line from a given external point to a circle of known radius and origin as the center

Please help

Answer 1

Let P1 be a 1 x 2 row vector of the external point's cartesian coordinates and P2 a similar vector of the circle's center coordinates. Let r be the circle's radius. Then Q1 and Q2 below will be the two possible points of tangency of a line from P1. Q1 is to the right and Q2 to the left as viewed from P1 looking toward P2.

 P = P1-P2;
 d2 = dot(P,P);
 Q0 = P2+r^2/d2*P;
 T = r/d2*sqrt(d2-r^2)*P*[0,1;-1,0];
 Q1 = Q0+T;
 Q2 = Q0-T;