Consider the curve .

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is an operator, meaning “the slope of the tangent of”. So the expression , meaning , is not a fraction.

In order words, it means the slope of the tangent of the curve at a point, such as point in the graph.

The symbol has no relation with the symbol . It means as shown in the graph. In other words,

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The symbol also has no relation with the symbol . It means the vertical distance between the current point , where , and the point on the tangent line , where is the slope of the tangent line. In other words,

or

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The relationship of and is that

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Similarly, for functions of 2 variables:

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For functions of 3 variables:

— Me@2018-07-15 09:30:29 PM

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2018.07.15 Sunday (c) All rights reserved by ACHK