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

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276 Mult<strong>ipl</strong>e and L<strong>in</strong>e Integrals [Ch. 7<br />

Example 3. (4x + 2y) dx+ (2x 6y) dy = dU. F<strong>in</strong>d U.<br />

Solution. Let A- O = O, y = Q. Then<br />

or<br />

U(x, y) =<br />

where C U (0, 0) is an arbitrary constant.<br />

Y<br />

Uo --4-<br />

XQ<br />

2<br />

+ 2y) dx + C= 3//<br />

Fig. 102<br />

/I<br />

X<br />

-i-c,<br />

4. Green's formula for a plane. If C is the boundary of a region S and<br />

the functions P (x, y) and Q (x, y) are cont<strong>in</strong>uous together with their firstorder<br />

partial derivatives <strong>in</strong> the closed region S-j-C, then Green's formula holds:<br />

(S)<br />

here t'^e circulation about the contour C is chosen so that the region S should<br />

rema<strong>in</strong> to the left.<br />

5. Applications of l<strong>in</strong>e <strong>in</strong>tegrals. 1<br />

) An area bounded by the closed contour C is<br />

= (p y dx= (p xdy<br />

(the direction of circulation of the contour is chosen counterclockwise).<br />

The fo low<strong>in</strong>g formula for area is more convenient for application:<br />

2) The work of a force, hav<strong>in</strong>g projections X = X(x, y, 2), Y = Y (x, y, z),<br />

Z (x, y t z) (or, accord<strong>in</strong>gly, the work of a force field), along a path C is

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