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Solução_Calculo_Stewart_6e

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F.<br />

400 ¤ CHAPTER 9 DIFFERENTIAL EQUATIONS<br />

TX.10<br />

3. (a) At t =0, there are about 300 rabbits and 100 foxes. At t = t 1, the number<br />

of foxes reaches a minimum of about 20 while the number of rabbits is<br />

about 1000. Att = t 2 , the number of rabbits reaches a maximum of about<br />

2400, while the number of foxes rebounds to 100. Att = t 3, the number of<br />

rabbits decreases to about 1000 and the number of foxes reaches a<br />

maximum of about 315. Ast increases, the number of foxes decreases<br />

greatly to 100, and the number of rabbits decreases to 300 (the initial<br />

populations), and the cycle starts again.<br />

(b)<br />

5.<br />

7.<br />

dW<br />

dR<br />

=<br />

−0.02W +0.00002RW<br />

0.08R − 0.001RW<br />

0.08 − 0.001W<br />

W<br />

dW =<br />

−0.02 + 0.00002R<br />

R<br />

⇔ (0.08 − 0.001W )RdW =(−0.02 + 0.00002R)W dR ⇔<br />

dR<br />

⇔<br />

0.08<br />

W<br />

<br />

− 0.001 dW = − 0.02 <br />

R<br />

+0.00002 dR<br />

⇔<br />

0.08 ln|W | − 0.001W = −0.02 ln|R| +0.00002R + K ⇔ 0.08 ln W +0.02 ln R =0.001W +0.00002R + K ⇔<br />

ln W 0.08 R 0.02 =0.00002R +0.001W + K ⇔ W 0.08 R 0.02 = e 0.00002R+0.001W +K ⇔<br />

R 0.02 W 0.08 = Ce 0.00002R e 0.001W<br />

⇔<br />

R 0.02 W 0.08<br />

e 0.00002R e 0.001W<br />

dy −ry + bxy<br />

= C. In general, if =<br />

dx kx − axy ,thenC =<br />

xr y k<br />

e bx e ay .<br />

9. (a) Letting W =0gives us dR/dt =0.08R(1 − 0.0002R). dR/dt =0 ⇔ R =0or 5000. SincedR/dt > 0 for<br />

0

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