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any idea ?
Pretty simple to see.
You can't add apples to pears.
M.Omax has unit N,
M.k1 has unit kN*mm
Those units are not the same, so you cannot add them, even if you squared the variables.
your first 3 moments are in N because 1/2 has no unit (it is a lenght?). Add a unit to 1/2 !