How to derive the formula to find electric flux through a cone or disc?

Dear student 
Here, the flux is equal to the product of the magnitude of the field and the total surface area. The flux will be given by,
Electric flux=Electric field x Area
Surface area of cone is given by,
πr ( l + r ) For   equilateral   cone   side   or   slant   height   ' l '   is   equal   to   the   diameter   of   the   base . i . e .   

l = 2 r

So ,   A = π   r ( 2 r + r ) 

3 r × πr = 3 πr ^2 Thus , Flux ,   ϕ = E × 3 πr^2