Solution to a 4th order equation

hey, i've become stuck on a question which asks for the general circularly symmetric solution to the equation:

Del^4 Psi = Del^2(Del^2 Psi) = 0 (Del is the grad operator)

The answer is Psi = A + Bln r + Cr^2 + Dr^2*ln r if that helps.

The question hints to use plane polar co-ordinates (r,phi)....but that kind of throws me because i've never done the Del^2 operator in plane polar co-ordinates....but I kind of get why it hints at that because you want circularly symmetric solutions. So any help would be much appriciated

cheers

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