SOLUTION: (s") + 6s' - 9s = t^2 where s" is like d^2x/dt^2 (s") + 6s' - 9s = t^2 where s" is like d^2x/dt^2

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Question 199856: (s") + 6s' - 9s = t^2
where s" is like
d^2x/dt^2
(s") + 6s' - 9s = t^2
where s" is like
d^2x/dt^2

Answer by jim_thompson5910(35256)   (Show Source): You can put this solution on YOUR website!
This is a 2nd order linear differential equation. So here are the steps to finding the general solution:


Step 1) Find the complementary solution

Solve the characteristic equation to get or . Since we have two unique real roots, this means that the complementary solution is


Note: recall, if you have two real roots and of the characteristic equation, then the complementary solution is


Step 2) Find the operator that annihilates the right hand side


The operator annihilates , , and


We can rewrite the given differential equation as:


When we apply the operator to both sides, we get:


Because annihilates , , and , we know that the particular solution is





Derive to get:






Derive again to get:




Now plug this information into the original differential equation to get











So this means that








Solve the system above to get , , and


So the particular solution is



This means that the general solution is





Plug in and (what we found earlier) to get:





So that is the final answer.



Note: there is another way to solve this problem, but it involves nasty integrals. Even though there's a lot going on here, the solution method is really straightforward.

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