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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Ravi prakash yadav
https://www.google.com/url?sa=t&source=web&rct=j&url=https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Book%253A_Electricity_and_Magnetism_(Tatum)/01%253A_Electric_Fields/1.06%253A_Electric_Field_E/1.6E%253A_Field_on_the_Axis_of_a_Uniformly_Charged_Disc&ved=2ahUKEwj40LuUgfbrAhXSmuYKHYYnA6oQFjADegQIAxAB&usg=AOvVaw3icBWX9N0q0MJdXQAFB_SU&cshid=1600545949340 see the derivation. now see the attachment,find the value of R using equation 1. and put R/2 again in the same formula to get the answer.