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TT2 #2

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Victor Ivrii:
(a) Determine the type of behavior (phase portrait) near the origin of the system of ODEs
\begin{equation*}
\left\{\begin{aligned}
&x'_t=y\ , \\
&y'_t=2x + y .
\end{aligned}\right.
\end{equation*}

(b) Solve for the system of ODEs from \textbf{2a} the initial value problem with $\ x(0)=2 \ ,\ y(0)=1\ $.

Yuan Bian:
x'= (0  1)
y'= (2  1)
r2-r-2=0
(r-2)(r+1)=0
r1=2, r2=-1
r1>0> r2
unstable saddle
(x)=c1e2t(1)+c2e-t(1)
(y)          (2)          (-1)
2b) c1+c2=2
      2c1-c2=1
so c1=1,c2=1
(x)=e2t(1)+e-t(1)
(y)       (2)     (-1)

Chang Peng (Eddie) Liu:
This is what I did... Please feel free to correct me if I'm wrong! :p

Chang Peng (Eddie) Liu:
2b

Yuan Bian:
different c1 and c2... ::)
and c2 is still in your final answer.

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