如果你愿意的话，你可以把书还回来，然后换取一些书店积分，你不能全额退款。
If you prefer, you can return the book and arrange for a store credit, you don't qualify for a refund.
这就是为什么，这个线积分，有着完美的定义，但却不能对它使用格林公式的原因。
And so that's why you have this line integral that makes perfect sense, but you can't apply Green's theorem to it.
不管哪种形式，都把线积分和二重积分联系在一起，来看看，能不能通过化简得到昨天的公式。
No matter which form it is, it relates a line integral to a double integral Let's just try to see if we can reduce it to the one we had yesterday.
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