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

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824 Chapter 25 Functions of several variables

The rate of change of area w.r.t. the radius is dA = 2πr. In practice functions often

dr

dependuponmorethanonevariable.Forexample,thevolume,V,ofacylinderdepends

upon the radius,r, and the height,h, and isgiven by

V = πr 2 h

whereV isthedependentvariable;r andhareindependentvariables.V isafunctionof

the two independent variables,r andh. We writeV =V(r,h).

Engineeringapplication25.1

Electricalpotentialinsideacathoderaytube

In recent years the cathode ray tube has declined in popularity as a screen display

device. It is now rarely used in consumer electronics as flat screen televisions have

becomemorepopular.However,itisstillsometimesusedinthespecialistelectronics

sector and instruments such as cathode ray oscilloscopes are still commonly in use

inmany laboratories.

Withinacathoderaytubetheelectricalpotential,V,willvarywithspatialposition

and time.Given Cartesiancoordinatesx,yandz, wecan write

V =V(x,y,z,t)

toshow thisdependence. Note thatV isafunctionoffourindependentvariables.

Engineeringapplication25.2

Powerdissipatedinavariableresistor

Thepower,P,dissipatedinavariableresistordependsupontheinstantaneousvoltage

acrossthe resistor, v, and the resistance,r. Itisgiven by

P = v2

r

Hence we may writeP =P(v,r) to show this dependence. The power is a function

of two independent variables.

Asanotherexampleofafunctionofmorethanonevariableconsiderathree-dimensional

surface as shown in Figure 25.1. The height,z, of the surface above thex--y plane depends

upon thexandycoordinates, that isz = z(x,y). If we are given values ofxand

y, then z(x,y) can be evaluated. This value of z is the height of the surface above the

point (x,y). We write, for example,z(3,−1) for the value ofzevaluated whenx = 3

and y = −1. The dependent variable, z, is a function of the independent variables

x andy.

Some important features are shown in Figure 25.1. The value of z at a maximum

point is greater than the values of z at nearby points. Point A is such a point. As you

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