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DryRun
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Homework Statement
I'm trying to understand the simplification of the general solution for homogeneous linear ODE with complex roots.
Homework Equations
In my notes, i have the homogeneous solution given as:
[tex]y_h (t)= C_1 e^{(-1+i)t}+C_2e^{(-1-i)t}[/tex]
And the simplified solution is given as:
[tex]y_h (t)= A e^{-t}\cos t+Be^{-t}\sin t[/tex]
The Attempt at a Solution
First, using Euler's formula, then I expanded each part individually before summing them all up:
[tex]C_1 e^{(-1+i)t}=C_1(e^{-t}(\cos t +i\sin t))=C_1e^{-t}\cos t +C_1e^{-t}i\sin t
\\C_2 e^{(-1-i)t}=C_1(e^{-t}(\cos t -i\sin t))=C_2e^{-t}\cos t -C_2e^{-t}i\sin t[/tex]
Now, adding these up, i just do not understand how the imaginary terms lose the "i" along the way. Can someone please clarify this part?
For the sake of completion, adding them up, i get:
[tex]C_1 e^{(-1+i)t}+C_2e^{(-1-i)t}
\\=C_1e^{-t}\cos t +C_1e^{-t}i\sin t+C_2e^{-t}\cos t -C_2e^{-t}i\sin t
\\=(C_1+C_2)e^{-t}\cos t + (C_1-C_2)e^{-t}i\sin t[/tex]where, [itex]A = (C_1+C_2)[/itex] and [itex]B=(C_1-C_2)[/itex]. However, the "i" coefficient of the sine term should not be there, according to the answer.
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