Since electric potential is a scalar quantity. So, the potential at the centre is given as V=K‌
q
R
+K‌
q
R
+....9 times V=‌
9Kq
R
...(i) Since, electric field is a vector quantity. So, the electric field cancel each other for the charges of opposite corner of polygon. Only 10q−(10−1)q=10q−9q=q will contribute the electric field at the centre of polygon. Thus, E=K‌
q
R2
....(i) From Eqs. (i) and (ii), we get ∴‌‌‌