Đội 1 :
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 số gần bằng) => A= 1,2124 (lấy số gần bằng)
=> A < 9/20
Đội 1 :
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 số gần bằng) => A= 1,2124 (lấy số gần bằng)
=> A < 9/20
Đặt $A=\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}$
\Rightarrow A=$\frac{1}{5}.(1+\frac{1}{3}+\frac{1}{5}+...+ \frac{1}{397})$
$=\frac{1}{5}.(1+\frac{1}{1+2}+\frac{1}{2+3}+...+ \frac{1}{198+199})$
$=\frac{1}{5}.(1+1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{198}-\frac{1}{199})$
$=\frac{1}{5}.(2-\frac{1}{199})$
$=\frac{397}{995}< \frac{9}{20}$
\Rightarrow A<$\frac{9}{20}$ ko biết có đúng ko bữa
Đặt $A=\frac{1}{5}+\frac{1}{15}+\frac{1}{25}+...+\frac{1}{1985}$
\Rightarrow A=$\frac{1}{5}.(1+\frac{1}{3}+\frac{1}{5}+...+ \frac{1}{397})$
$=\frac{1}{5}.(1+\frac{1}{1+2}+\frac{1}{2+3}+...+ \frac{1}{198+199})$ $=\frac{1}{5}.(1+1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{198}-\frac{1}{199})$
$=\frac{1}{5}.(2-\frac{1}{199})$
$=\frac{397}{995}< \frac{9}{20}$
\Rightarrow A<$\frac{9}{20}$ ko biết có đúng ko bữa