You are just quoting the general result for energy stored in a capacitor with charge Q and voltage V. The question asks, for a capacitor already charged at potential V0, what is the increase in energy caused by adding a small amount of extra charge? These are not the same thing.
To see why, use the graph given in question to extend the line shown by increasing charge (on x-axis) by a small amount. The area under this additional section corresponds to the extra energy stored. Can you work out a formula for the approximate area of this additional section in terms of the extra charge ΔQ?
To see why, use the graph given in question to extend the line shown by increasing charge (on x-axis) by a small amount. The area under this additional section corresponds to the extra energy stored. Can you work out a formula for the approximate area of this additional section in terms of the extra charge ΔQ?
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