For iit jeee there is no fixed weitage, it vary from year to year...
f 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
The section-wise breakup of marks in the past four years JEE has been given below for your reference.
'Electrostatics and Current Electricity', and 'Electromagnetism'-each of these topics has contributed about 15% to the total marks
'Optics' too contributes approximately 13% to the IIT-JEE
In the past four years, approximately 20% of the total marks pertain to Mechanics. Within Mechanics, the favorite topics seem to be 'Rigid Body Rotation' and 'Motion on a Plane'.
'Mechanical Waves' (sound) has accounted for approximately 12% of the total marks.
'Organic Chemistry' has the highest weightage with mean marks of 36%. Topics like 'Isomerism' and 'Name Reactions', and 'Stability of reaction intermediates' should be thoroughly covered.
'Physical Chemistry' is a close second with a mean weightage of 33%.
'Inorganic Chemistry' and 'General Chemistry' account for 21% and 10% of the total marks. Inorganic Chemistry should be prepared by studying the group properties and periodicity.
'Calculus' accounts for about 30% of the total marks in JEE Screening.
'Algebra' has a weightage of about 23% of the total marks.
'Analytical Geometry' has a weightage of about 20% of the total marks.
The topics of 'Vectors and 3-D Geometry' and 'Probability' have a weightage of approximately 6% each.
'Trigonometry' carries a weightage of about 9%. It has been observed that this weightage was as high as 17% in JEE
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