H2_pic2

import geometry;

size(9cm, 0);
defaultpen(fontsize(10pt));
dotfactor = 6;
point A,B,C,O;
real R=2;
O=origin; A=-2R*dir(30);
circle circ=circle(O, R);
line[] t=tangents(circ, A);

point[] Tp=intersectionpoints(circ, t[1]);
B=Tp[0];
point[] Tp=intersectionpoints(circ, t[0]);
C=Tp[0];

markrightangle(size=0.3cm, A,B,O);
markrightangle(size=0.3cm, O,C,A);
markangle(Label("$70^\circ$"), radius=0.5cm, C,A,B);

draw(A--(C+dir(A--C))^^A--(B+dir(A--B)));
draw(B--O^^O--C);
draw(circ, black+1.2pt);

dot(O^^A^^B^^C);
label("$A$", A, S);
label("$B$", B, NW);
label("$C$", C, S);
label("$O$", O, NNE);