`S=1/(2^2)-1/(2^4)+1/(2^6)-...+1/(2^(4n-2)2^(4n))+...+1/(2^2018)-1/(2^2020)`

`S=1/(2^2)-1/(2^4)+1/(2^6)-...+1/(2^(4n-2))-1/(2^(4n))+...+1/(2^2018)-1/(2^2020)`

`2^2S=2^2(1/(2^2)-1/(2^4)+1/(2^6)-...+1/(2^(4n-2))-1/(2^(4n))+...+1/(2^2018)-1/(2^2020))`

`4S=1-1/(2^2)+1/(2^4)-1/(2^6)+...-1/(2^(4n-2))+1/(2^(4n))-...-1/(2^2018)`

`4S+S=(1-1/(2^2)+1/(2^4)-1/(2^6)+...-1/(2^(4n-2))+1/(2^(4n))-...-1/(2^2018))+(1/(2^2)-1/(2^4)+1/(2^6)-...+1/(2^(4n-2))-1/(2^(4n))+...+1/(2^2018)-1/(2^2020))`

`5S=1-1/(2^2020)`

`=>S=(1-1/(2^2020)):5`

`=>S=1:5-1/(2^2020):5`

`=>S=1/5-1/(2^2020) .1/5`

`=>S=1/5-1/(2^2020. 5)<1/5`

`=>S<0,2(đpcm)`

sửa 7749 lần :)))

cậu đợi tí =)) t sửa lại ehee

uaa=)) v là t bị lừaaa

mình ghi sai ạ

`-5/2-(0,01)^4 . 50^4+(-3/2)^20:(-3/2)^18`

`=-5/2-(0,01.50)^4+(-3/2)^2`

`=-5/2-(0,5)^4+9/4`

`=(-5/2+9/4)-0,065`

`=(-10/4+9/4)-13/200`

`=-1/4-13/200`

`=-50/200-13/200`

`=-63/200`