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42.83 = (22)2.(23)3 = 24.29 = 213
93.272 = (32)3.(33)2 = 36.36 = 312
82.253 = (23)2.(52)3 = 26.56 = (2.5)6 = 106
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Ta có: \(\frac{1}{3^2}< \frac{1}{2.3}\)
\(\frac{1}{4^2}< \frac{1}{3.4}\)
.....................
\(\frac{1}{2014^2}< \frac{1}{2013.2014}\)
\(\Rightarrow A< \frac{1}{2^2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2013.2014}\)
Đặt \(B=\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2013.2014}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2013}-\frac{1}{2014}\)
\(=\frac{1}{2}-\frac{1}{2014}< \frac{1}{2}\)
\(\Rightarrow A< \frac{1}{2^2}+\frac{1}{2}=\frac{3}{4}\)
\(\text{Ta có: }n^2>n^2-1=\left(n-1\right)\left(n+1\right)\)
\(\Rightarrow\frac{1}{n^2}< \frac{1}{\left(n-1\right)\left(n+1\right)}=\frac{1}{2}\left(\frac{1}{n-1}-\frac{1}{n+1}\right)\)
\(A=\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+....+\frac{1}{2014^2}< \frac{1}{1.3}+\frac{1}{2.4}+\frac{1}{3.5}+...+\frac{1}{2013.2015}\)
\(=\frac{1}{2}\left(1-\frac{1}{3}\right)+\frac{1}{2}\left(\frac{1}{2}-\frac{1}{4}\right)+\frac{1}{2}\left(\frac{1}{3}-\frac{1}{5}\right)+...+\frac{1}{2}\left(\frac{1}{2013}-\frac{1}{2015}\right)\)
\(=\frac{1}{2}\left(1-\frac{1}{3}+\frac{1}{2}-\frac{1}{4}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{2013}-\frac{1}{2015}\right)\)
\(=\frac{1}{2}\left(1+\frac{1}{2}-\frac{1}{2014}-\frac{1}{2015}\right)\)
\(=\frac{1}{2}\left(\frac{3}{2}-\frac{1}{2014}-\frac{1}{2015}\right)\)
\(=\frac{3}{4}-\frac{1}{2}\left(\frac{1}{2014}+\frac{1}{2015}\right)< \frac{3}{4}\)
Vậy .............
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a) 27 . 75 + 25 .27 - 150 = 27 . (75 + 25) - 150
= 270 . 100 - 150
= 27 000 - 150
= 26 850
b) 3.52 - 16 : 22 = 12,25 - 16 : 4
= 12,25 - 4
= 8,25
c) 20 - [30 - (5 - 1)2 ] = 20 - [30 - 42 ]
= 20 - 30 - 16
= (-10) - 16
= -26
d) 60 : {[(12 - 3) . 2] + 2} = 60 : {[9 . 2] + 2}
= 60 : {18 + 2}
= 60 : 20
= 3
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1. 3A = 3^2 + 3^3 + 3^4 + ... + 3^100 + 3^ 101
=> 3A - A = (3^2 + 3^3 + 3^4 + ... + 3^100 + 3^ 101) - (3 + 3^2 + 3^3 + 3^4 + ... + 3^100 )
=> 2A = 3^101 - 3 => 2A + 3 = 3^101 vậy n = 101
2. 2A = 8 + 2 ^ 3 + 2^4 + ... + 2^20 + 2^21
=> 2A - A = (8 + 2 ^ 3 + 2^4 + ... + 2^20 + 2^21) - (4+ 2^2 + 2 ^ 3 + 2^4 + ... + 2^20 )
=> A = 2^21 là một lũy thừa của 2
3.
a) 3A = 3 + 3^2 + 3^3 + 3^4 + ... + 3^100 + 3^ 101
=> 3A - A = (3 + 3^2 + 3^3 + 3^4 + ... + 3^100 + 3^ 101) - (1 + 3 + 3 ^2 + 3 ^ 3 + ... + 3 ^100)
=> 2A = 3^101 - 1 => A = (3^101 - 1)/2
b) 4B = 4 + 4 ^ 2 + 4 ^3 + 4 ^ 4 + ... + 4 ^ 100 + 4^ 101
=> 4B - B = (4 + 4 ^ 2 + 4 ^3 + 4 ^ 4 + ... + 4 ^ 100 + 4^ 101) - (1 + 4 + 4 ^ 2 + 4 ^3 + 4 ^ 4 + ... + 4 ^ 100 )
=> 3B = 4^101 - 1 => B = ( 4^101 - 1)/2
c) xem lại đề ý c xem quy luật như thế nào nhé.
d) 3D = 3^101 + 3^ 102 + 3^ 103 + ... + 36 150 + 3^ 151
=> 3D - D = (3^101 + 3^ 102 + 3^ 103 + ... + 36 150 + 3^ 151) - (3 ^100 + 3 ^ 101 + 3 ^ 102 + .... + 3 ^ 150)
=> 2D = 3^ 151 - 3^100 => D = ( 3^ 151 - 3^100)/2
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Ko ghi đề
\(2A=2+2^2+...+2^{101}\\ 2A-A=2^{101}-1\\ =>A=2^{101}-1\)
Mấy cái khác cg lm như v (b thì 3b)
Nhớ đúng mk nhá
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Bài 1:
a,\(A=3+3^2+3^3+...+3^{2010}\)
\(=\left(3+3^2+3^3+3^4\right)+....+\left(3^{2007}+3^{2008}+3^{2009}+3^{2010}\right)\)
\(=3\left(1+3+3^2+3^3\right)+....+3^{2007}\left(1+3+3^2+3^3\right)\)
\(=3.40+...+3^{2007}.40\)
\(=40\left(3+3^5+...+3^{2007}\right)⋮40\)
Vì A chia hết cho 40 nên chữ số tận cùng của A là 0
b,\(A=3+3^2+3^3+...+3^{2010}\)
\(3A=3^2+3^3+...+3^{2011}\)
\(3A-A=\left(3^2+3^3+...+3^{2011}\right)-\left(3+3^2+3^3+...+3^{2010}\right)\)
\(2A=3^{2011}-3\)
\(2A+3=3^{2011}\)
Vậy 2A+3 là 1 lũy thừa của 3
aC = a + a^2 + a^3 + .... + a^n+1
( a - 1 )C = ( a + a^2 + a^3 + ... + a^n+1 ) - ( 1 + a + a^2 + a^3 + ... + a^n )
( a - 1 )C = an+1 - 1
C =an+1 - 1 / a
a^(n+1)-1