零加一中：不用直尺，只用圆规找出线段 AB中点 解答 
Let R = AB
From A draw acr1 with r = R.
From B draw arc2 with r = R.
arc1 and arc2 meet at C.
From C draw arc3 with r = R.
arc3 and arc2 meet at D.
From D draw arc4 with r = R.
arc4 and arc2 meet at E.
E is on AB and EB = R.
From E draw arc5 wirg r = 2R.
arc1 and arc5 meet at F.
From F draw arc6 with r = R.
arc6 meets AB at M.
∆EFA ~ ∆FAM by AA.
They are both isosceles with same base angles.
∆EFA has 3 sides 2R, 2R, R.
∆EFA should have 3 sides R, R, R/2.