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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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25 Functionsofseveral

variables

Contents 25.1 Introduction 823

25.2 Functionsofmorethanonevariable 823

25.3 Partialderivatives 825

25.4 Higherorderderivatives 829

25.5 Partialdifferentialequations 832

25.6 TaylorpolynomialsandTaylorseriesintwovariables 835

25.7 Maximumandminimumpointsofafunctionoftwovariables 841

Reviewexercises25 846

25.1 INTRODUCTION

Inengineeringtherearemanyfunctionswhichdependuponmorethanonevariable.For

example,thevoltageonatransmissionlinedependsuponpositionalongthetransmission

lineaswellastime.Theheight ofliquidinatankdepends upon theflowratesintoand

outof the tank. We shall examine some more examples inthis chapter.

Wehavealreadydiscusseddifferentiationoffunctionsofonevariable.However,we

alsoneedtobeabletodifferentiatefunctionsoftwoormorevariables.Thisisachieved

by allowing one variable to change at a time, holding the others fixed. Differentiation

under these conditions iscalledpartial differentiation.

25.2 FUNCTIONSOFMORETHANONEVARIABLE

InChapter10wesawhowtodifferentiateafunctiony(x)withrespecttox.Manystandard

derivatives were listed and some techniques explained. Sinceyis a function ofx

wecallythedependentvariableandxtheindependentvariable.Thefunctionydepends

upon the one variablex. Consider the following example.

The area,A, ofacircle depends only upon the radius,r, and isgiven by

A(r) = πr 2

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