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\(=\frac{1}{5}.\left(\frac{1}{1}+\frac{1}{3}+\frac{1}{5}+...+\frac{1}{397}\right)\)
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\
K nhé
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
Đặt A=1/3 + 1/5 +1/7 +1/9+.....+1/243
A=1/3 +(1/5+1/7+1/9)+(1/11+1/13+1/15+....+1/27)+(1/29+1/31+1/33+.......+1/81)+(1/83+1/85+1/87+...+1/243)
=> A>1/3+ 1/9 x3+1/27 x9+1/81x27+ 1/243x81=1/3x5=5/3
=> A>5/3>5/4
=>A>5/4
=> 1/3+1/5+1/7+.....+1/397 > 5/4
=>1+1/3+1/5+1/7+.....+1/397 > 9/4
=>1/5x (1+1/3+1/5+1/7+.....+1/397)> 9/4 x 1/5
=>1/5+1/15+1/25+......+1/1985 > 9/20