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Title [Harim Yoo] Chapter 1.3, #26 (강의 7) - I don't understand!
The Essential Guide to Competition Math (Fundamentals) 7
Author alo*** Date Posted 2024-01-06 오후 12:11:36

Hi,


I'm having difficulty understanding how to solve Chapter 1.3, #26 (from 강의 7).

Why do we suddenly divide (2021x2020) by 2, then multiply by 2 again? Could you please clarify and explain thoroughly and with detail? I feel like your explanation in the video was a little rushed. And how would combination/permutation apply to this question?


Thank you for your time.
2024-01-08 오후 12:30:54

Hi!!

 

Here is what's up. First, we choose two circles out of 2021 circles.

This is how we get 2021 choose 2, which is (2021×2020)÷2.

Now, for any pair of two circles chosen, we always get 2 points of intersection. Think about two rings intertwined with one another.

 

Hence, for each pair, we always get two points of intersection! Hope this helps. Yay!!

 

 
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