ABSTRACT
Let https://www.w3.org/1998/Math/MathML"> f : D ⊂ R 2 → R https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003449652/afdcc6b9-9be3-4ad4-883c-989795168de8/content/math6_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> be a function defined on an open set D containing https://www.w3.org/1998/Math/MathML"> ( a , b ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003449652/afdcc6b9-9be3-4ad4-883c-989795168de8/content/math6_2.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> . Then https://www.w3.org/1998/Math/MathML"> ∂ f ∂ x ( a , b ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003449652/afdcc6b9-9be3-4ad4-883c-989795168de8/content/math6_3.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> and https://www.w3.org/1998/Math/MathML"> ∂ f ∂ y ( a , b ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003449652/afdcc6b9-9be3-4ad4-883c-989795168de8/content/math6_4.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> measure the rate of change of f in the direction of x and y axis respectively.
