german polizie to the rescue.........

challo jus givin u the rough method.............

divide the cone into discs of thickness dx.........let us define the function of electric field due to a disc at a distance of x from the disk having a radius R as Q*F(x,R).........where Q= charge of the disc..........

now finding F(x,R) is the donkey job that ull have to do patiently...............(again integrating using thin rings).....

now for this thin disc Q= q*pi (R(x))²dx / (1/3pi r²H)................where r is the base radius of the cone............and R(x) is the Radius of the cylinder as a function of x........(which ull have to find again)

so we have F(x,R)q*pi (R(x))²dx / (1/3pi r²H)....where a= D to b= D+H...........

the integrated result will give u the E at a distance of D from the apex of the cone...........

sorry yaar im in a hurry...........so jus give the brief mtd of solving the problem.........