Finding an annihilator operator

[bcjochim07]

1. The problem statement, all variables and given/known data
Find an annihilator operator for (cosx)^2


2. Relevant equations



3. The attempt at a solution
first derivative -2cosxsinx
second derivative 2(sinx)^2 - 2(cosx)^2
third derivative 4sinxcosx +4cosxsinx

This isn't getting me anywhere (D^2 + 2) is close but I still have the 2(sinx)^2 term.

Any suggestions?

[Dick]

What's wrong with (D^3+4*D)? It's staring you in the face.

[bcjochim07]

Of course! I see it now. Thanks.