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

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210 0 CHAPTER 14 PARTIAL DERIVATIVES<br />

27. y cosx = x 2 + y 2 , so let F(x, y) = ycosx - x 2 - y 2 = 0. Then by Equation 6<br />

dy<br />

dx<br />

-y sin x - 2x 2x + y sin x<br />

= --:-----'"----<br />

cos x - 2y cosx- 2y .<br />

and<br />

[2xy - (1 + y 2 ) (1 + x-iy 2 )]/(1 + x 4 y 2 ) _ (1 + y 2 )(1 + x 4 y 2 ) - 2xy<br />

(x2 - 2xy(1 + x4y 2 ))/(1 + x4y 2 ) - x 2 - 2xy(1+ x4y2)<br />

- 1 + x4y2 + y2 + x4y4 - 2xy<br />

- x 2 - 2xy - 2x 5 y3<br />

31. x 2 + 2y 2 + 3z 2 = 1, so let F(x, y, z) = x 2 + 2y 2 + 3z 2 - 1 = 0. Then by Equations 7<br />

f)z = .:_ Fx = _ 2x = -~ and oz = _ Fy = _ 4y = _ 2y<br />

ox Fz 6z 3z oy Fz QZ 3z .<br />

_ • oz F. . -yz yz<br />

33. e· = x yz, so let F(x, y, z) = e· - xyz = 0. Then~ = - -F = --.-- = --- and<br />

. ux 'z e· - xy ez - xy<br />

8z<br />

oy<br />

F 11 - xz xz<br />

- Fz = --e=---x-y = -ez ___ x_y<br />

35. Since x andy are eac~ functions oft, T(x, y) is a function oft, so by the Chain Rule, ~ = ~: + ~~~;.After<br />

dx 1<br />

3 seconds, x = .Jl+t = .,;r+3 = 2, y = 2 + ~ t = 2 + H 3) = 3, -d = .J1+t<br />

t 2 1 + t<br />

1 -.!. and dy -.!.<br />

2 v'1 + 3 - 4, dt - 3 .<br />

Then~~ = Tx(2, 3) ~~ + Ty(2, 3) ~; = 4(t) + 3(~ ) = 2. Thus the temperature is rising at a rate of2°C/ s.<br />

ac ac .<br />

37. C = 1449.2 + 4.6T- 0.055T 2 + 0.00029T 3 + 0.016D, so 8T = 4.6 - O.llT + 0.00087T 2 and oD = 0.016.<br />

According to the graph, the diver is experiencing a temperature of approximately 12.5°C at t = 20 minutes, so<br />

. c:;, = 4.6 - 0.11(12.5) + 0.00087(12.5) 2 ~ 3.36. By sketching tangent lines_ at t = 20 to the graphs given, we est~ate<br />

dD 1 dT 1 . dC 80 dT 80 dD 1 1 .<br />

dt ~ '2 and (jj ~ - 10<br />

. Then, by the Cham Rule, (jj = &T (jj + &Ddt ~ (3.36) (-Iii) + (0.016) ( 2) r::::: -0.33.<br />

Thus the spe<strong>ed</strong> of sound experienc<strong>ed</strong> by the diver is decreasing at a rate of approximately 0.33 m/s per minute.<br />

39. (a) V = l!wh, so by the Chain Rule,<br />

dV = 8V dl! + 8V dw + oV dh = wh dl! + l!h dw + l!w dh = 2 . 2 . 2 + 1 . 2 . 2 + 1 . 2 . ( _ 3) = 6 m3/s.<br />

dt 81! dt aw dt &h dt dt dt dt<br />

© 2012 Ccngagc Learning. All Rights RescrYcd. May not be scann<strong>ed</strong>. copi<strong>ed</strong>, or duplicat<strong>ed</strong>. or, post<strong>ed</strong> toP publicly accessible website, in whole or in part.

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