Question Number 151884 by mathdanisur last updated on 23/Aug/21
$$\mathrm{f}\left(\mathrm{x}\right)=\left(\mathrm{x}-\mathrm{1}\right)\left(\mathrm{x}-\mathrm{2}\right)…\left(\mathrm{x}-\mathrm{2021}\right) \\ $$$$\mathrm{f}\:^{'} \left(\mathrm{2021}\right)\:=\:? \\ $$
Answered by mr W last updated on 24/Aug/21
$$\mathrm{ln}\:{y}=\mathrm{ln}\:\left({x}−\mathrm{1}\right)+\mathrm{ln}\:\left({x}−\mathrm{2}\right)+…+\mathrm{ln}\:\left({x}−\mathrm{2021}\right) \\ $$$$\frac{{y}'}{{y}}=\frac{\mathrm{1}}{{x}−\mathrm{1}}+\frac{\mathrm{1}}{{x}−\mathrm{2}}+…+\frac{\mathrm{1}}{{x}−\mathrm{2021}} \\ $$$${y}'=\left({x}−\mathrm{1}\right)\left({x}−\mathrm{2}\right)…\left({x}−\mathrm{2021}\right)\left[\frac{\mathrm{1}}{{x}−\mathrm{1}}+\frac{\mathrm{1}}{{x}−\mathrm{2}}+…+\frac{\mathrm{1}}{{x}−\mathrm{2021}}\right] \\ $$$${y}'\mid_{{x}=\mathrm{2021}} =\mathrm{2020}×\mathrm{2019}×…×\mathrm{2}×\mathrm{1}=\mathrm{2020}! \\ $$
Commented by mathdanisur last updated on 24/Aug/21
$$\mathrm{Thank}\:\mathrm{you}\:\boldsymbol{\mathrm{S}}\mathrm{er} \\ $$
Answered by Olaf_Thorendsen last updated on 23/Aug/21
$${f}\left({x}\right)\:=\:\underset{{k}=\mathrm{1}} {\overset{\mathrm{2021}} {\prod}}\left({x}−{k}\right) \\ $$$${f}\left({x}\right)\:=\:\left({x}−\mathrm{2021}\right)\underset{{k}=\mathrm{1}} {\overset{\mathrm{2020}} {\prod}}\left({x}−{k}\right) \\ $$$$\mathrm{Let}\:{g}\left({x}\right)\:=\:\underset{{k}=\mathrm{1}} {\overset{\mathrm{2020}} {\prod}}\left({x}−{k}\right) \\ $$$${f}\left({x}\right)\:=\:\left({x}−\mathrm{2021}\right){g}\left({x}\right) \\ $$$${f}'\left({x}\right)\:=\:\left({x}−\mathrm{2021}\right){g}'\left({x}\right)+{g}\left({x}\right) \\ $$$${f}'\left(\mathrm{2021}\right)\:=\:{g}\left(\mathrm{2021}\right)\:=\:\underset{{k}=\mathrm{1}} {\overset{\mathrm{2020}} {\prod}}\left(\mathrm{2021}−{k}\right) \\ $$$${f}'\left(\mathrm{2021}\right)\:=\:\underset{{k}=\mathrm{1}} {\overset{\mathrm{2020}} {\prod}}{k}\:=\:\mathrm{2020}! \\ $$
Commented by mathdanisur last updated on 23/Aug/21
$$\mathrm{Thank}\:\mathrm{You}\:\boldsymbol{\mathrm{S}}\mathrm{er} \\ $$