25S = 1 - 1/52+1/54- 1/56+.......+1/52008- 1/52010
Cộng 2 vế với S ta có :
26S = 1 - 1/52012 < 1 suy ra S< 1/26
\(5^2.S=1-\frac{1}{5^2}+\frac{1}{5^4}-.....+\frac{1}{5^{2008}}-\frac{1}{5^{2010}}\)
\(25S=1-\frac{1}{5^2}+\frac{1}{5^4}-...+\frac{1}{5^{2008}}-\frac{1}{5^{2010}}\)Cộng 2 vế với S ta có
\(26S=1-\frac{1}{5^{2012}}\)\(\Rightarrow26S< 1\Rightarrow S< \frac{1}{26}\)
đăng cho vui, ko cần giải cx đc
S= \(\frac{1}{5^2}\) -\(\frac{1}{5^4}\) +\(\frac{1}{5^6}\)-.......+ \(\frac{1}{5^{2010}}\)- \(\frac{1}{5^{2012}}\)
\(\Rightarrow\) 52S= 1- \(\frac{1}{5^2}\) +\(\frac{1}{5^4}\) -..........+\(\frac{1}{5^{2008}}\)- \(\frac{1}{5^{2010}}\)
25S= 1- \(\frac{1}{5^2}\)+ \(\frac{1}{5^4}\)-..........+\(\frac{1}{5^{2008}}\)- \(\frac{1}{5^{2010}}\)
\(\Rightarrow\)25S+ S= 1- \(\frac{1}{5^2}\)+ \(\frac{1}{5^4}\)-..........+\(\frac{1}{5^{2008}}\)- \(\frac{1}{5^{2010}}\)+ \(\frac{1}{5^2}\)- \(\frac{1}{5^4}\)+\(\frac{1}{5^6}\)-.........+ \(\frac{1}{2^{2010}}\)- \(\frac{1}{5^{2012}}\)
26S= 1- \(\frac{1}{5^{2012}}\)< 1
\(\Rightarrow\)S< \(\frac{1}{26}\)
Trl :
Bạn kia làm đúng rồi nhé !
Học tốt nhé bạn @
1+1= ??? ai biết không