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

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208_Functions of Several Variables_[Ch. 6<br />

Example 4. The function z of the arguments x and y is def<strong>in</strong>ed by the<br />

equations<br />

_. , dz , dz<br />

F<strong>in</strong>d ^- and 3- .<br />

ox dy<br />

Solution. First method. By differentiation we f<strong>in</strong>d three equations that<br />

connect the differentials of all five variables:<br />

dx = du + dv ,<br />

From the first two equations we determ<strong>in</strong>e du and dv:<br />

20 dx dy , _ dy 2u dx<br />

"^ 2(v u) '<br />

2(u w)<br />

Substitut<strong>in</strong>g <strong>in</strong>to the third equation the values of du and rfy just found, we<br />

have:<br />

Whence<br />

dy<br />

d<br />

6wu (u<br />

=-'<br />

= 3- 3au, 3- =TT (w-fy).<br />

v ;<br />

^jc dt/ 2<br />

Second method. From the third given equation we can f<strong>in</strong>d<br />

*=3Jf + 3^; f = 3'Jf + 3t,'f!. (5)<br />

^ dx dx dy dy l<br />

^ '<br />

dy<br />

Differentiate the first two equations first with respect to x and then with<br />

respect to y:<br />

From the first system we f<strong>in</strong>d<br />

From the second system we f<strong>in</strong>d<br />

f<br />

.<br />

dx dy dy<br />

v dv<br />

du___<br />

__ u<br />

dx~~ v a' dx~ u v<br />

da = _l__ dv_ 1<br />

dy~~2(u v)' dy~~2(vu)'<br />

Substitut<strong>in</strong>g the expressions ~ and ~ <strong>in</strong>to formula (5), we obta<strong>in</strong><br />

'

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