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

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194 Functions of Several Variables (Ch. 6<br />

where f(P) and / (P,) are values of the function at the po<strong>in</strong>ts P and<br />

If the function z is differentiate, then the follow<strong>in</strong>g formula holds:<br />

where a is the angle formed by the vector / with the x-axis (Fig. 67).<br />

Y<br />

e^<br />

P(*,y)<br />

Fig. 67<br />

In similar fashion we def<strong>in</strong>e the derivative <strong>in</strong> a given direction / for a<br />

function of three arguments u = f(x, y t z). In this case<br />

du du du du<br />

, , = cos a + 5- cos p + 3- cos '<br />

r<br />

v,<br />

dl dx dy<br />

dz<br />

T (2)<br />

where a, P, Y are ^ ne angles between the direction / and the correspond<strong>in</strong>g<br />

coord<strong>in</strong>ate axes. The directional derivative characterises the rate of change<br />

of the function <strong>in</strong> the given direction.<br />

Example 1. F<strong>in</strong>d the derivative of the function z = 2x 2<br />

3

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