The point where three equal lines can be drawn to the circumference is the centre

http://books.google.com.hk/books?id=Z9g2AAAAMAAJ&pg=PA60&lpg=PA60&dq=three+equal+line+from+circumference&source=bl&ots=0-gYM6ZlEI&sig=FUeJhFp94TTWnCizYPa3CXZ3zw0&hl=zh-TW&sa=X&ei=m6xYVOrcMYXPmwXehoGACA&ved=0CDcQ6AEwAw#v=onepage&q=three equal line from circumference&f=false
睇唔明個prove最尾個位