A doubt on total differentials

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The discussion centers on understanding total differentials in the context of a horizontal plane through point P. The rectangle PDEF is confirmed to be flat and not sloped, which is crucial for the analysis. Participants are encouraged to clarify their statements for better feedback. The question posed is whether the short proof regarding the relationship between partial derivatives and small changes in dimensions accurately defines the total differential. Overall, the conversation emphasizes the importance of clear communication and the correct interpretation of mathematical concepts.
Rishabh Narula
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Please refer to the images and my handwritten note(i tried my best handwriting) for the question
 

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There is a horizontal plane (z constant) through the point P. They are identifying a rectangle in it as PDEF. There is nothing there that one can argue with. It is not "sloped". With that in mind, you should reconsider your thinking.

PS. If you type your statements in, then it is easier for people to comment on specific parts of it.
 
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Thanks,will do that next time onwards.Is the short proof right,that the different partials multiplied by small changes in different dimensions,when added up give the total small change?
 
That is what is defined as the "total differential".
 
Question: A clock's minute hand has length 4 and its hour hand has length 3. What is the distance between the tips at the moment when it is increasing most rapidly?(Putnam Exam Question) Answer: Making assumption that both the hands moves at constant angular velocities, the answer is ## \sqrt{7} .## But don't you think this assumption is somewhat doubtful and wrong?

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