sir,

i’d like to know about the weightage set for all the topics in iitjee .

It is not possible to discuss all the important concept here. you should ask the question in which you feel problem.and i willl try to solve your problem,for iit jeee there is no fixed weitage, it  vary from year to year...
if you will see the pattern of last year papers,then you will find that question were framed in such a way that it include more than one  chaptre. and in whole paper almost entire syllabus are covered .so it is not advisable to leave any topic.

But the important topics are.

Maths:     

calculus, Vectors, Complex numbers, co-ordinate geometry(2D+3D) Quadratic equations.

Physics:   

Optics, Modern physics, Thermodynamics, A/C circuits, Fluid mechanics, Experiments.

Chemistry:

Solid state, chemica bonding, Aldehydes, ketones, carboxylic acids, Alkenes,alkynes, stereo and geometrical isomerism, transition elements, coordiantion compounds,electro chemistry, kinetics and equilibrium.

but I will suggest you to cover alll the topics and give more stress on above given topics

gud luck

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regards

Ramesh