In this paper, some counter examples to the conclusions of singular solution and envelope in two textbooks of ordinary differential equation are given.
给出了两种常微分方程教材中关于奇解与包络关系结论的反例。
They became even more popular in Britain when a card could be posted in an unsealed envelope for one half-penny - half the price of an ordinary letter.
当邮寄非密封的卡片只要半便士（普通信件邮费的一半）时，邮寄贺卡变得异常流行。
They became even more popular in Britain when a card could be posted in an unsealed envelope for one half-penny - half the price of an ordinary letter.
当邮寄非密封的卡片只要半便士（普通信件邮费的一半）时，邮寄贺卡变得异常流行。
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