CMR : 1/5 + 1/15 + 1/25 +....+1/1985 <9/20
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CMR: 1/5+1/15+1/25+...+1/1985 <9/20
A = (1/5)+(1/15)+(1/25)+...+(1/1985)=
1/5+1/3*5+1/5*5+1/7*5+.........+1/397*5
5A=1+1/3+1/5+1/7+.......+1/397
5A-1=1/3+1/5+1/7+.......+1/397
Đặt B= 1/3+1/5+1/7+.......+1/397
=>.......................Tính đc B=5,06241 (lấy gần bằng)=> A= 1,2124 (lấy số gần bằng)
=> A < 9/20
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CMR:\(\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}< \frac{9}{20}\)
CMR:
\(\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}< \frac{9}{20}\)
CMR \(A=\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}< \frac{9}{20}\)
CMR: \(\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}<\frac{9}{20}\)
CMR:
1/5 +1/15 +1/25 +...+1/1985 < 9/20
CMR \(\frac{1}{5}+\frac{1}{15}+\frac{1}{25}\) +..................+\(\frac{1}{1985}<\frac{9}{20}\)
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Cho : A= \(\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}\)
CMR: A<\(\frac{9}{20}\)
A=1/5+1/15+1/25+...+11/985
A=1/5.(1+1/3+1.5+...+1/397)
=1/5.(1+1/1+2+1/2+3+...+1/198+199)
=1/5.(1+1−1/2+1/2−1/3+...+1/198−1/199)
=1/5.(2−1/199)
=397/995<920\
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CMR \(\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}<\frac{9}{20}\)
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