Since the capacitor causes a 90 degrees phase shift at the cutoff frequency then the power is half, not the voltage. The voltage is -3db (0.707 times) when the power is half.
To answer the question clearly (of course you know the answer), you need an unequivocal definition of involved formula
entities, if they are are scalars or vectors? It's not clear from your notation in my opinion.
Generally, you can apply the usual calculus for DC networks to AC, if you change to complex respectively vectorial entities. But the connotation of "ohm" (e.g. "ohmic resistance") is contraditing the usage of the term "ohms law" in this regard, I think.
Of course, I agree, a simple formula like the one I gave in my response requires some explanation - in case the reader does not know anything about electronics basics and the rules to manipulate with imaginary figures.
However, don´t you think also the simple statement "Ohm´s law does not apply for ac networks" deserves some explanation? Otherwise PPP1262 could believe that the voltage-to-current ratio wouldn´t be constant in ac circuits (if the frequency remains constant).
For your information, here are two excerpts from
Horowitz/Hill "The art of electronics":
1.) "As we will see, it is possible to generalize Ohm´s law, replacing the word "resistance" with "impedance", in order to describe any circuit containing linear passive devices (resistors, capacitors, inductors)."
2.) "...we can apply complex Ohm´s law to circuits containing capacitors and inductors, just as for resistors, once we know the reactance of a capacitor or inductor".
Your error was, that you didn´t consider the complex nature of these reactances and the rules to manipulate them mathematically.