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B=3+3^2+3^3+..+3^2009+3^2010
=(3+3^2+3^3)+...+(3^2008+3^2009+3^2010)
=3(1+3+3^2)+..+3^2008(1+3+3^2)
=13(3+...+3^2008) chia hết cho 13
a, 5M = 5+1+1/5+1/5^2+.....+1/5^2011
4M=5M-M=(5+1+1/5+1/5^2+.....+1/5^2011)-(1+1/5+1/5^2+.....+1/5^2012)
= 5-1/5^2012
=> M = (5 - 1/5^2012)/4
Tk mk nha
a)M = 1 + 3 + 32 +....+ 3118 + 3119
M = (1 + 3 + 32)+(33+34+35)+...+(3117+3118+3119)
M = 1x(1+3+9)+33x(1+3+9)+...+3117x(1+3+9)
M = 1x13+33x13+...+3117x13
M = 13x(1+33+...+3117)
Vậy M chia hết cho 13
Xét N :
N = \(\frac{1}{2.2}\)+\(\frac{1}{3.3}\)+\(\frac{1}{4.4}\)+...+\(\frac{1}{2009.2009}\)+\(\frac{1}{2010.2010}\)
Ta có :
\(\frac{1}{2.2}\)< \(\frac{1}{1.2}\)
\(\frac{1}{3.3}\)< \(\frac{1}{2.3}\)
...
\(\frac{1}{2009.2009}\)<\(\frac{1}{2008.2009}\)
\(\frac{1}{2010.2010}\)<\(\frac{1}{2019.2010}\)
Cộng vế theo vế của các bất đẳng thức trên , ta có :
\(\frac{1}{2.2}\)+\(\frac{1}{3.3}\)+\(\frac{1}{4.4}\)+...+\(\frac{1}{2009.2009}\)+\(\frac{1}{2010.2010}\) < \(\frac{1}{1.2}\)+\(\frac{1}{2.3}\)+...+\(\frac{1}{2008.2009}\)+\(\frac{1}{2019.2010}\)
=> N < 1 - \(\frac{1}{2}\)+\(\frac{1}{2}\)-\(\frac{1}{3}\)+...+\(\frac{1}{2009}\)-\(\frac{1}{2010}\)
=> N < 1 - \(\frac{1}{2010}\)<1
=> N < 1
Ta có:
\(\frac{1}{3^2}< \frac{1}{2.3}\)
\(\frac{1}{4^2}< \frac{1}{3.4}\)
\(\frac{1}{5^2}< \frac{1}{4.5}\)
....
\(\frac{1}{100^2}< \frac{1}{99.100}\)
\(\Rightarrow\frac{1}{3^2}+\frac{1}{4^2}+\frac{1}{5^2}+\frac{1}{6^2}+...+\frac{1}{100^2}< \frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{99.100}\)\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}\)
\(-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{99}-\frac{1}{100}=\frac{1}{2}-\frac{1}{100}< \frac{1}{2}\)
=> đpcm
1/2^2 + 1/3^2 +... + 1/100^2 < 1/4 + 1/2.3 + 1/3.4 +... + 1/99.100
1/2^2 + 1/3^2 +... + 1/100^2 < 1/4 + 1/2 - 1/3 + 1/3 -1/4 +... + 1/99 + 1/100
1/2^2 + 1/3^2 +... + 1/100^2 < 1/4 + 1/2 - 1/100
1/2^2 + 1/3^2 +... + 1/100^2 < 3/4 - 1/100 < 3/4 (đpcm)