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a=1/2^2+1/2^4+1/2^6+.....+1/2^100,b=12^2^100.Tính a/b
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a/ \(A=\dfrac{6}{5.8}+\dfrac{22}{8.19}+\dfrac{24}{19.31}+\dfrac{198}{101.200}\)
\(=2\left(\dfrac{3}{5.8}+\dfrac{11}{8.19}+\dfrac{12}{19.31}+...+\dfrac{99}{101.200}\right)\)
\(=2\left(\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{19}+....+\dfrac{1}{101}-\dfrac{1}{200}\right)\)
\(=2\left(\dfrac{1}{5}-\dfrac{1}{200}\right)\)
\(=\dfrac{39}{100}\)
b/ \(A=\dfrac{1}{2^2}+\dfrac{1}{4^2}+.....+\dfrac{1}{100^2}\)
Ta có :
\(\dfrac{1}{2^2}< \dfrac{1}{1.2}\)
\(\dfrac{1}{4^2}< \dfrac{1}{3.4}\)
...........
\(\dfrac{1}{100^2}< \dfrac{1}{99.100}\)
\(\Leftrightarrow A< \dfrac{1}{1.2}+\dfrac{1}{2.3}+....+\dfrac{1}{99.100}\)
\(\Leftrightarrow A< 1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+.....+\dfrac{1}{99}-\dfrac{1}{100}\)
\(\Leftrightarrow A< 1-\dfrac{1}{100}< 1\left(đpcm\right)\)
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A=1-2+3-4+...+99-100 SSH=(100-1):1+1=100 Sh
=>A=(1-2)+(3-4)+....+(99-100)
vì chia thành cặp suy ra 100:2 =50 cặp
A=(-1)+(-1)+...(-1)
A=(-1).50
A=-50