I used the formula Current=charge density× e × area × drift velocity i.e. i=neAV So this yield that drift velocity inversely proportional to area of cross section But the answer to this question is that drift velocity don't depend on cross section Please explain why can't I use this formula
Please explain why can't I use this formula
You still have an unknown, the current $i$, in your formula.
You need to use some other knowledge to eliminate $i$ and get the velocity in terms of the independent quantities in your problem (the voltage and the wire geometry) before you can determine whether the drift velocity depends on any particular quantity.
You can use the formula but for a fixed voltage and conductor length, $I$ and $A$ are not independent variables.
For a given length of conductor, the resistance of the conductor is inversely proportional to the cross sectional area. Then, per Ohm’s law, for a fixed voltage the current is proportional to the cross sectional area, per your equation.
For a given amount of charge per unit volume, the drift velocity $V$ is proportional to the current density, or $I/A$. Since for a fixed voltage if $A$ increases $I$ increases and if $A$ decreases $I$ decreases, the drift velocity remains constant.
Hope this helps.