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Exercicios resolvidos James Stewart vol. 2 7ª ed - ingles

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238 0 CHAPTER 14 PARTIAL DERIVATIVES<br />

(b) Here u = ( 12• )?.). so by Equation 14.6.9, Du T(6,4) = VT(6,4) · u = T, (6,4) ~ + Tv(6,4) ~·Using our<br />

estimates from part (a), we have Du T(6, 4) l':;j (3.5) ~ + ( -3.0) ~ = ~ l':;j 0.35. This means that as we move<br />

through the point (6, 4) in the direction ofu , the temperature increases at a rate of approximately 0.35°C/m.<br />

. r(6+h~,4+h~)-T(6,4 )<br />

Alternatively, we can use Definition 14.6.2: Du T( 6, 4) = lim ,<br />

h-o<br />

h<br />

which ·we can estimate with h = ± 2 ,;2. Then Du T(6, 4) ~ T( 8 , 6 ) ~T( 6 ,<br />

4 ) =<br />

80 - ~O = 0,<br />

2 2 2v2<br />

D T(6 4)<br />

T(4, 2) - T(6,4) 74-80 3 . ( ) 3 0 I<br />

- 2v2 -2v2 v2<br />

u , ~ tn = ~ = tn' Averagmg these values, we have D u T 6, 4 ~ 27z ~ 1.1 C m.<br />

() T. ( ) _ !_ [T. ( )] - lim T,('x, y +h) - T:r.(x,y) T. ( 6 4 ) _lim T., (6,4 +h) - T.,(6, 4) 1 . h<br />

C :V!/ X 1 y - a x X 1 y - h , SO xy 1 - h W liC We Can<br />

. y h-o , h-o<br />

estimate with h = ±2. We have Tx (6, 4) ~ 3.5 from part (a), but we will also ne<strong>ed</strong> values for T.,(6, 6) and T,(6, 2). If we<br />

use h = ±2 and the values given in the table, we have<br />

T,( 6<br />

, 6<br />

) l':;j T(8, 6) - T(6, 6) = 80 - 75 = 2<br />

. 5<br />

, T, ( 6 , 6<br />

) ~ T(4, 6)- T (6, 6) = 68 - 75 = 3<br />

. 5<br />

.<br />

2 2 -2 -2<br />

Averaging these values, we estimate T.,(6, 6) ~ 3.0. Similarly,<br />

Tx( 6 , 2<br />

) l':;j T(8, 2) ~ Tx(6, 2) = 90 ; 87 = l.S, T.,( 6 , 2 ) ~ T(4, 2) ~<br />

2 T(6 , 2) = 74 ~ 87 = 6 . 5 .<br />

Averaging these values, we estimate T, (6, 2) ~ 4.0. Finally, we esti~ate T., 11 (6, 4):<br />

T. (6 4) ~ T.,(6, 6)- T.,(6, 4) _ 3.0 - 3.5 _ _ 0 25 T. ( 6 4 ) ~ T.,(6, 2)- T.,(6, 4) _ 4.0 - 3.5 __ 0 25<br />

:vy ! ~ 2 - 2 - • > X)J 1 ~ -2 - -2 - • •<br />

Averaging these values, we have Txy (6, 4) ~ -0.25.<br />

13. f(x, y) = (5y 3 + 2x 2 y) 8 => f.,= 8(5y 3 + 2x 2·yf(4xy) = 32xy(5y 3 + 2x 2 y) 7 ,<br />

fv = 8(5y 3 + 2x 2 yf (15y 2 + 2x 2 ) = (16x 2 + 120y 2 )(5y 3 + 2x 2 y) 7<br />

17. S(u,v,w) = uarctan(vyfw)<br />

S w = u · 1<br />

+ (:JW) 2 (v · ~ w - 1 1 2 )<br />

1 ufo<br />

S,. = arctan(vy'W), Su = u · 1<br />

+ (vfo)2 (fo) = 1 +v2w'<br />

= 2Vw (~: v2w)<br />

19. f(x,y) = 4x 3 - x y 2 => f x = 12x 2 - y 2 , f v = -2xy, f.,., = 24x, fv 11 = - 2_:~: , fxv = fvx = -2y<br />

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