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\(S=1+2+2^2+2^3+...+2^9\)
Đặt \(2S=2+2^2+2^3+2^4+...+2^{10}\)
\(2S-S=2^{10}-1\) hay \(S=2^{10}-1< 2^{10}\)
\(\Rightarrow\) \(2^{10}=2^2.2^8< 5.2^8\)
Vậy \(S< 5.2^8\)
\(#Tuyết\)
2S=2+2^2+...+2^10
=>S=2^10-1=1023
5*2^8=256*5=1280
=>S<5*2^8
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
35+335+3335+....+333...35(có 100 c/s 3)
=35+35+3x100+35+3x1100+....+3x111...00(99 c/s 1)+35
=35x100+3x(100+1100+...+111....100(99 c/s 1))
=3500+3x(1.100+11.10+....+111...1(99 c/s 1).10)
=3500+3 x ((1+11+..+111...1(99 c/s 1))x100)
Chịu thôi , đến đoạn này tớ bí !!!
Sorry nha!
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![](https://rs.olm.vn/images/avt/0.png?1311)
A = 1 . 3 . 5 - 3. 5 . 7 + 5 . 7 . 9 - 7.9.11 + ... + 97.99.101
4A = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + … + 98.99.100.4
= 1.2.3.4 + 2.3.4(5 - 1) + 3.4.5(6 - 2) + … + 98.99.100(101 - 97)
= 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + … + 98.99.100.101 - 97.98.99.100
= 98.99.100.101
=> A = 98.99.25.101
A = 24 497 550
![](https://rs.olm.vn/images/avt/0.png?1311)
mk ko viết đề nhé
C=1.3.5.8 + 3.5.7.8 + 5.7.9.8 + … + 95.97.99.8
C= 1.3.5(7 + 1) + 3.5.7(9 - 1) + 5.7.9(11 - 3) + … + 95.97.99(101 - 93)
C= 1.3.5.7 + 15 + 3.5.7.9 - 1.3.5.7 + 5.7.9.11 - 3.5.7.9 + … + 95.97.99.101 - 93.95.97.99
C= 15 + 95.97.99.101
=>C=\frac{15 + 95.97.99.101}{8}
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)