称之为区域R上fdA的二重积分,会向大家解释这些符号的含义的。
So, we'll call that the double integral of our region, R, of f of xy dA and I will have to explain what the notation means.
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.
当计算二重积分时，要多了解这些符号的具体含义。
OK, we'll come up with more concrete notations when we see how to actually compute these things.
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