- #1
sa1988
- 222
- 23
I'm working on a pretty simple ODE but am getting really confused about one little bit.
It's of this form:
y'' + y = t
So, in solving the complementary solution I use the characteristic method to find:
λ2 + 1 = 0
Hence λ = ± i
Therefore:
yc = Aei t + Be-i t
This expands to:
yc = Acos(t) + i Asin(t) + Bcos(t) - i Bsin(t)
BUT
If I run the same thing through DSolve in Mathematica, I get:
yc = Asin(t) + Bcos(t)
There's a clear similarity between that and my own answer, but also a very clear difference!
Where have I gone wrong? Where are the imaginary terms in the Mathematica solution? Plus, even if I ignore or cancel the imaginary terms in my solution I end up with (A+B)cost(t) which is still wrong, it seems.
What's gone wrong?
Thanks!
It's of this form:
y'' + y = t
So, in solving the complementary solution I use the characteristic method to find:
λ2 + 1 = 0
Hence λ = ± i
Therefore:
yc = Aei t + Be-i t
This expands to:
yc = Acos(t) + i Asin(t) + Bcos(t) - i Bsin(t)
BUT
If I run the same thing through DSolve in Mathematica, I get:
yc = Asin(t) + Bcos(t)
There's a clear similarity between that and my own answer, but also a very clear difference!
Where have I gone wrong? Where are the imaginary terms in the Mathematica solution? Plus, even if I ignore or cancel the imaginary terms in my solution I end up with (A+B)cost(t) which is still wrong, it seems.
What's gone wrong?
Thanks!
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