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

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

2. Higher-order differentials. The second differential of a function<br />

z f(x t y) is the differential of the differential (first-order) of this function:<br />

We similarly def<strong>in</strong>e the differentials of a function z of order higher than<br />

two, for <strong>in</strong>stance:<br />

and, generally,<br />

d*z = d(d*z)<br />

dz = d(d n - l<br />

z).<br />

If z = /(x, y), where x and y are <strong>in</strong>dependent variables, then the second<br />

differential of the function z is computed from the formula<br />

Generally, the follow<strong>in</strong>g symbolic<br />

formula holds true:<br />

the b<strong>in</strong>omial law.<br />

it is formally expanded by<br />

If z = f (x, (/), where the arguments x and y are functions of one or several<br />

<strong>in</strong>dependent variables, then<br />

"- * + 2<br />

If x and i/ are <strong>in</strong>dependent variables, then d 2 jt = 0, d 2<br />

y = Q, and formula (2)<br />

becomes identical with formula (!)<br />

Example 2. F<strong>in</strong>d the total differentials of the first and second orders of<br />

the function<br />

z = 2;t 2<br />

Solution. First method. We have<br />

Therefore,<br />

Further we have<br />

dz = fr<br />

whence it follows that<br />

3xyy 2 .<br />

*-

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