![](https://rs.olm.vn/images/avt/0.png?1311)
Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{1}{5}+\frac{1}{20}+\frac{1}{44}+...+\frac{1}{170}=\frac{2}{10}+\frac{2}{40}+\frac{2}{88}+...+\frac{2}{340}\)
\(\frac{2}{2x5}+\frac{2}{5x8}+\frac{2}{8x11}+...+\frac{2}{17x20}=2x\left(\frac{1}{2x5}+\frac{1}{5x8}+\frac{1}{8x11}+...+\frac{1}{17x20}\right)\)
\(2x\left(\frac{1}{2}-\frac{1}{5}+\frac{1}{5}-\frac{1}{8}+\frac{1}{8}-\frac{1}{11}+...+\frac{1}{17}-\frac{1}{20}\right)\)=2x\(\left(\frac{1}{2}-\frac{1}{20}\right)\)
2x\(\frac{9}{20}=\frac{9}{10}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Nó dài lắm bạn à!!!!
Bó tay luôn!!!!
Đúng thì k nha!!!!
![](https://rs.olm.vn/images/avt/0.png?1311)
Câu C giải rồi
\(B=\dfrac{1}{5}+\dfrac{1}{20}+\dfrac{1}{44}+\dfrac{1}{77}+\dfrac{1}{119}+\dfrac{1}{170}+\dfrac{1}{230}+\dfrac{1}{299}\)
\(=2\left(\dfrac{1}{10}+\dfrac{1}{40}+\dfrac{1}{88}+\dfrac{1}{154}+\dfrac{1}{238}+\dfrac{1}{340}+\dfrac{1}{460}+\dfrac{1}{598}\right)\)
\(=\dfrac{2}{3}\left(\dfrac{3}{2.5}+\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}+\dfrac{3}{14.17}+\dfrac{3}{17.20}+\dfrac{3}{20.23}+\dfrac{3}{23.26}\right)\)
\(=\dfrac{2}{3}\left(\dfrac{1}{2}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{8}+...+\dfrac{1}{23}-\dfrac{1}{26}\right)\)
\(=\dfrac{2}{3}\left(\dfrac{1}{2}-\dfrac{1}{26}\right)=\dfrac{4}{13}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có A=1/5+1/20+1/44+...+1/170
=>A=1/10+1/40+1/88+...+1/340
A=\(\frac{1}{2.5}\)+\(\frac{1}{5.8}\)+\(\frac{1}{8.11}\)+......+\(\frac{1}{17.20}\)
A=1/3.(1/2-1/5+1/5-1/8+1/8-1/11+.....+1/17-1/20)
A=1/3.(1/2-1/20)
A=\(\frac{3}{20}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Bài 1 :
Đặt A=1.2+2.3+3.4+4.5+.........+99.100
=> 3A=1.2.3+2.3.(4-1)+........+99.100.(101-98)
3A=1.2.3+2.3.4-1.2.3+........+99.100.101-98.99.100
3A=99.100.101
A=33.100.101
A=333300
Bài 2 :
1:20 + 1:44 + 1:77 + 1:119 + 1:170 = \(\frac{1}{20}+\frac{1}{44}+\frac{1}{77}+\frac{1}{119}+\frac{1}{170}=\frac{1}{10}=0,1\)
1)1.2+2.3+3.4+4.5+...+99.100
đặt 3D=1.2+2.3+3.4+...+99.100
=1.2.3+2.8.3+...+3.4.3+4.5.3+...+99.100.3
=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+99.100.(101-98)
=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5
=99.100.101
=999900
D=999900:3=333300
nếu đúng nhớ cảm ơn nhak. mình ko bít làm bài 2