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Mathematics Optional Group UPSC 2021

How was the difficulty of the UPSC 2020 mains paper? Has anyone finished solving the entire paper? 
I have solved to some extent, but don't know the difficulty level of certain topics which I have not finished revising and hence do not remember the formulas and associated concepts. 

Among the topics that I have finished revising, I found one problem from numerical analysis section difficult, (the one which has three point lagrange interpolating polynomial and limit for expression of derivative). The rest looked doable/calculation based and I have solved most of them. Apart from that, from the topics that I have not revised yet, modern algebra looks quite easy/ straightforward theory (I studied Algebra in college as I am a math graduate)


Specifically, I have not finished revising PDE, complex analysis, modern algebra, dynamics/statics and mechanics. (I do not intend to study fluid dynamics and will leave it) How was the difficulty level of the paper in these topics? Were the problems mechanical/ from IMS notes or did they require innovation/tricks outside the book?

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I have completed whole syllabus once ~ last year. I have given CSE prelims once (2020), but couldn't get through. So, I am currently preparing for CSE 2021. (I am a math graduate and graduated in 2019 (Btech in math and computation))

However, I find that the time for revision is a bit less as the prelims exam was held late this year and also there is only about 2.5 months between pre and mains. I have completed only 7/13 topics, and it is February already. I plan to start prelims specific preparation from 1 Feb onwards. Also, I have not done mains answer writing or ethics or essay preparation. Given my scenario, what do you feel should be the way forward for me?
I am contemplating one of 3 paths currently -
1. Drop CSE ambition for this year and prepare for CSE 2022, by focusing on completing math
and mains specific preparation for now instead of pre .
2. Try to give some time (say 1 hour) daily, even during prelims preparation.
3. Try to take out 1/2 to 1 month out of intensive pre preparation and devote the time to finish math/answer writing.
Any help will be greatly appreciated. 
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@AlexanderSupertramp Thanks a lot. I'll check it out. Do you have math optional too?


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@Celeborn No, I graduated from IIT Delhi, Mathematics and Computation Department (2019 Batch).  
Thank you for taking the time out to answer my question. I will follow this strategy.

@Davidjones  Thank you for answering my question. I did not solve the entire paper properly, as I have not finished revising all the topics, but I found the questions doable in the papers. As I said, I have completed the syllabus once and faintly remember some topics like Algebra, Complex Analysis and Mechanics/Statics/Dynamics.

However, I have not solved all the questions till the final answer. Could you please point out which questions from the paper were difficult/ what topics had out of the box questions? It will help me to save some time, so that I can try specifically those questions. 

Though the Lagrange interpolating polynomial question 8(b) does look difficult. I couldn't solve this in my first attempt. Also, the question contains Fluid dynamics (Q8c), so I wouldn't have attempted it anyway.

So, maybe time was an issue? I have never written a full math optional paper before, and don't know how much time it takes to complete the whole paper. Thanks for the guidance. I appreciate the help.
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The question about lagrange ho jaata bhai if thoda time leke sochte tum. FD ka question was easy ek baar attempt karte tum kyuki difgerntiate karne ke baad easy 0 nikal jaata and y component bhi easily aa jaata. Moment of inertia ka question was real time consumer aur last me answer nahi aaya. Similarly i attempted simplex ka question udher i think meri calculation mistakes hui hai. Bhai i feel second paper mera acha gaya compare kare to. First paper sab easy bol rahe hai but i ll be frank, meri bhut galti hui hai. Hindsight me even i feel paper was easy par i lost time in some questions and baaki questions jaldi me attempt hue. Bhai solve the complete paper with 3 hours deadline. Also kuc environmrnt bana do tab do exam. You will gwt a good idea

Nahin, I have not covered FD (and don't plan to cover it in future also), thats why I said I would leave it. 
So, I understand from your comment that time management is important during paper. Hence, practice seems necessary for math paper also. I will give previous year paper (2019 or 2018) like mock test in constrained time now, because I have already seen many questions of 2020 paper and even solved some of them completely, that I thought looked unusual. So, perhaps, if I give the paper, it will not be a good indicator of how well I can write a fresh paper in constrained time. By the way, do you recommend solving test series like IMS, Dias or Shankar, before the actual paper? I was thinking of just solving previous year papers in constrained time like mock test.

Thanks a lot for the advice. Its pretty useful.

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