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We have a converse if F is defined in the entire plane.
如果F定义在整个平面上，反过来成立。
youdao
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F: Feel free to drop by my office if you have a question.
F：如果有问题，请不要客气，随时到我的办公室来。
youdao
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F: Feel free to drop by my office if you have a question.
F：如果有问题，请不要客气，随时到我的办公室来。
youdao