In the Kelvin scale, which has no negative figures, the symbol for degrees is not used after the number. So the freezing point of water is 273k, not 273 ~ K.
2
在屏幕的一侧会出现一个符号，而在另一侧，会显示圆圈内的两个符号。
On one part of the screen, a symbol would appear, and on the other side two symbols inside a circle were shown.
3
为了提醒我们是在封闭曲线上做积分，经常在积分符号上加个圆圈，告诉我们，这条曲线自我封闭。
To remind ourselves that we are doing it along a closed curve, very often we put just a circle for the integral to tell us this is a curve that closes on itself.
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