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Delayed Derivative Operator


Hi,
 I would like to be able to have an equation

eq := D[f[x],x]==5;

And a substitution rule

rule = f[x]->x^3;

And perform the substitution

eq/.rule

And get back the equation 3x^2==5. But Mathematica sees f'[x] and
doesn't do any substitutions. I tried a delayed assignment for eq but
it didn't work. What can I do?

-Thanks



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