code for figure

> org:=[0,0]: cc:=[1,0]: thta:=Pi/6.:
bb:= [cos(thta),0]: aa:=[cos(thta),sin(thta)]:
tri:=plot({[org,cc],[bb,aa],[org,aa]},thickness=3,
color=black):
circ:=plot([cos(t),sin(t),t=0..Pi/6],thickness=3):
circ2:=plot([.4*cos(t),.4*sin(t),
t=0.. .9*Pi/6],thickness=3):
hd2:=plot([ [ .4*cos(Pi/6*.9), .4*sin(Pi/6*.9)],
[ .4*cos(Pi/6*.9), .3*sin(Pi/6*.9)]],thickness=3):
txt:=plots[textplot]({[.4,.3,`1`],[.75,.25,`sin(theta)`],
[1.05,.3,`theta`],[.3,.1,`theta`],[bb[1],bb[2]-.02,`B`],
[cc[1],cc[2]-.02,`C`],
[-.05,0,`O`],[aa[1]+.05,aa[2]+.05,`A`]},
font=[TIMES,BOLD,16]):
plots[display]({txt,hd2,circ2,circ,tri}, axes=none):

>