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6+6.9+6.92+6.93+..............+6.999
\(=6.\left(1+9+9^2+9^3+...........+9^{99}\right)\)
Đặt A=1+9+92+93+........+999
\(\Rightarrow9A=9+9^2+9^3+9^4+.....+9^{100}\)
\(\Rightarrow9A-A=\left(9+9^2+9^3+9^4+.....+9^{100}\right)-\left(1+9+9^2+9^3+............+9^{99}\right)\)
\(\Rightarrow8A=9^{100}-1\)
\(\Rightarrow A=\frac{9^{100}-1}{8}\)
+)Thay A vào \(6.\left(1+9+9^2+9^3+...........+9^{99}\right)\)được:
\(=6.\frac{9^{100}-1}{8}=\frac{54^{100}-6}{8}\)
Chúc bn học tốt
![](https://rs.olm.vn/images/avt/0.png?1311)
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Ta có : 28.34 + 68.86
<=> 28.34 + 2.34.86
<=> 28.34 + 172.34
<=> 34.(28+172)
<=> 34.200 = 6800
![](https://rs.olm.vn/images/avt/0.png?1311)
2 . 125 . 18 + 36 . 252 + 4 . 233 . 9
=250 . 18 + 36 . 252 + 4 . 233 . 9
=4500 + 36 . 252 + 4 . 233 . 9
=4500 + 9072 + 4 . 233 . 9
= 13572 + 4 . 233 . 9
=13576 . 233 . 9
= 3163208 . 9
=28468872
k nhà mik làm bài này mệt lắm đó!!!!!!!! phù phù huhu
2.125.18+36.252+4.233.9 = 36.125+36.252+36.233
= 36(125+252+233)
= 36.610
= 21960
k cho mk nha.
![](https://rs.olm.vn/images/avt/0.png?1311)
3.25.8+4.37.6+2.38.12
=24.25+24.37+24.38
=24.(25+37+38)
=24.100
=2400
chúc bạn học tốt :)!
3.25.8+4.37.6+2.38.12
=24.25+24.37+24.38
=24.(25+37+38)
=24.100=2400
mik nha
![](https://rs.olm.vn/images/avt/0.png?1311)
\(=\dfrac{1}{3}\left(\dfrac{1}{3}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{9}+...+\dfrac{1}{15}-\dfrac{1}{18}\right)\)
\(=\dfrac{1}{3}\left(\dfrac{1}{3}-\dfrac{1}{18}\right)=\dfrac{1}{3}\cdot\dfrac{5}{18}=\dfrac{5}{54}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(S=\dfrac{1}{3.5}+\dfrac{1}{5.7}+\dfrac{1}{7.9}+...+\dfrac{1}{97.99}\)
\(2S=\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{97.99}\)
\(2S=\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{97}-\dfrac{1}{99}\)
\(2S=\dfrac{1}{3}-\dfrac{1}{99}\)
\(2S=\dfrac{32}{99}\)
\(S=\dfrac{32}{99}:2\)
\(S=\dfrac{16}{99}\)
Cảm ơn bạn rất nhiều! Bạn đã cứu mình rùi! Xin trân thành cảm ơn!
E=1/1.3+1/3.6+1/6.9+.............+1/20.23
<=>E=1/1-1/3+1/3-1/6+1/6-1/9+...........+1/20-1/23
<=>E=1/1-1/23
<=>E=22/23
Kb và k mk nha mn.
Cho hai phan so 1/n va 1/n+1 (n thuoc z)chung to rang h cua hai phan so nay bang hieu cua chung