Doubt from binomial theorem

solve the 2nd question…
Please anyone solve ASAP!
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Bruh even I didn’t get this question while attempting this paper…

You’ve gotta observe that it’s the multiplication of (1+x)^20*(1+x)^20 series. So that will yield (1+x)^40. Which in turn gives us the summation 40Cr. We know that nCr maximum value lies at nCn/2.
Thus r=20. This was what PNM sir (Resonance) did. I’ve not found any other approach.

How is it the multiplication of (1+x)^20 * (1+x)^20???

Arey multiplying first with last term of each expression.

watch from 36:23
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Haa. I got it…

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When you the same terms as given in question and add you see that they are the coefficients of X^R and so now to compare you need find coefficient of X^R in (1+x)^40 which is 40CR…
And hence its maximum is r=20…!

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