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.
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。
He is most famous for the play Death of a Salesman, which was put on stage in 1949 and won both a Pulitzer Prize and a Drama Critics Circle Award.
他的代表作《推销员之死》于1949年面世，曾获得普利策奖和戏剧评论界奖。
If you feel you haven't learn it you can put a circle around the item and add in to the list for a future day.
假如你感觉你不能够学到，你就围个圆圈在这个项目上并把他加在名单上以后解决。
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