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Problems in Mathematical Analysis.pdf - pwp.net.ipl.pt

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Sec. 9] Differentiation of Implicit Functions 207<br />

Solution.<br />

we obta<strong>in</strong><br />

First method. Differentiat<strong>in</strong>g both equations with respect to #<br />

du . dv<br />

whence<br />

Similarly we f<strong>in</strong>d<br />

?ff __ u ~^~y dv<br />

__ u-\-x<br />

dx~~ x y '<br />

dx~~ x y '<br />

du _ v + y dv _<br />

dy~~ x y '<br />

dy~~~ x y<br />

*<br />

Second method. By differentiation we f<strong>in</strong>d two equations that connect the<br />

differentials of all four variables:<br />

Whence<br />

du 4- dv = dx + dy,<br />

x du + u dx + y dv + v dy =4).<br />

Solv<strong>in</strong>g this system for the differentials du and dv, we obta<strong>in</strong><br />

_<br />

dx x y '<br />

(}y__-f-x<br />

dx~~xy<br />

4. Parametric representation<br />

'<br />

t)f/ x y f<br />

dv _ v -}-x<br />

dy~~x y<br />

'<br />

xy<br />

of a function. If a function z of the variables<br />

x and y is represented parametrically by the equations<br />

and<br />

z = z(u, v)<br />

then the differential of this function may be found from the follow<strong>in</strong>g system<br />

of equations<br />

dx , , dx<br />

.<br />

dx 5~du-\--5- dv,<br />

du dv<br />

dz<br />

xdu<br />

dz<br />

dv<br />

.<br />

dv.<br />

Know<strong>in</strong>g the differential dz^p d* + qdy, we f<strong>in</strong>d the partial derivatives<br />

dz . dz<br />

^~=pr and 3- ~^. ^<br />

dx<br />

dy

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