The answer to if it is necessary to take iit jam coaching is NO. There is absolutely no need to take any coaching for preparing for IIT JAM exams but then it also depends and varies for every student. The IIT JAM exam depends on a few things which are-

Building good concepts
Practising quality questions
Staying consistent

If a student can achieve these, then they can easily crack the IIT JAM exam. There is absolutely no rocket science in this. All you need are good books, right study material for IIT JAM and lots of dedication because if you decide to prepare firmly with dedication for something, nothing can stop you from achieving that.
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