code for figure

> curv:=plots[spacecurve]({[sin(t),cos(t),cos(t)+2],
[sin(t),cos(t),0]},t=0..2*Pi,thickness=3,color=blue):
lines:=plots[polygonplot3d]({[[0,0,2],[1,0,2]],[[0,0,2],
[cos(Pi/3.),sin(Pi/3.),sin(Pi/3)+2]],[[0,0,0],[1,0,0]],
[[0,0,0],[cos(Pi/3.),sin(Pi/3.),0]],[[0,0,-.5],
[0,0,3]]},thickness=2):
txta:=plots[textplot3d]({[1.3,0,0,`[a,0,0]`]},
font=[TIMES,BOLD,12],color=black):
txtb:=plots[textplot3d]([.5,0,0,`theta`],
font=[TIMES,BOLD,12],color=black):
txtp:=plots[textplot3d]([ 1.1*cos(Pi/3.),1.1*sin(Pi/3.),1.1*(sin(Pi/3)+2) ,`P`],font=[TIMES,BOLD,12],color=black):
plots[display]({txtp,txtb,txta,lines,curv},
axes=none,tickmarks=[0,0,0],orientation=[-21,65]);