If you look at the angles around the fold, you find that if you number the angles in a circle, all the even-numbered angles add up to a straight line, all the odd-numbered angles add up to a straight line.
如果观察折痕周围的角， 你会发现在数围成一圈的角时， 所有列为偶数的角加起来是一条直线。 所有列为奇数的角加起来是一个直线。