is the absence of free electrons, the only reason for insulators to prevent charge flow? we know that two plates of capacitor are at different voltages with charge on them. But charges don’t flow in air. Why

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Sum of series 2[n/2] ! [n/2]!/ n! [2Co-2C2+3C2....+[-1]^n[n+1]2Cn] where n is an even positive integer. Kindly provide solution of this question and how to generalise this. How do we know that which expansion will give this?

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