Hãy nhập câu hỏi của bạn vào đây, nếu là tài khoản VIP, bạn sẽ được ưu tiên trả lời.
Bài 3:
a,Đặt A = \(\frac{1}{2}-\frac{1}{4}+\frac{1}{8}-\frac{1}{16}+\frac{1}{32}-\frac{1}{64}\)
A = \(\frac{1}{2}-\frac{1}{2^2}+\frac{1}{2^3}-\frac{1}{2^4}+\frac{1}{2^5}-\frac{1}{2^6}\)
2A = \(1-\frac{1}{2}+\frac{1}{2^2}-\frac{1}{2^3}+\frac{1}{2^4}-\frac{1}{2^5}\)
2A + A = \(\left(1-\frac{1}{2}+\frac{1}{2^2}-\frac{1}{2^3}+\frac{1}{2^4}-\frac{1}{2^5}\right)+\left(\frac{1}{2}-\frac{1}{2^2}+\frac{1}{2^3}-\frac{1}{2^4}+\frac{1}{2^5}-\frac{1}{2^6}\right)\)
3A = \(1-\frac{1}{2^6}\)
=> 3A < 1
=> A < \(\frac{1}{3}\)(đpcm)
b, Đặt A = \(\frac{1}{3}-\frac{2}{3^2}+\frac{3}{3^3}-\frac{4}{3^4}+...+\frac{99}{3^{99}}-\frac{100}{3^{100}}\)
3A = \(1-\frac{2}{3}+\frac{3}{3^2}-\frac{4}{4^3}+...+\frac{99}{3^{98}}-\frac{100}{3^{99}}\)
3A + A = \(\left(1-\frac{2}{3}+\frac{3}{3^2}-\frac{4}{4^3}+...+\frac{99}{3^{98}}-\frac{100}{3^{99}}\right)-\left(\frac{1}{3}-\frac{2}{3^2}+\frac{3}{3^3}-\frac{4}{3^4}+...+\frac{99}{3^{99}}-\frac{100}{3^{100}}\right)\)
4A = \(1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}-\frac{100}{3^{100}}\)
=> 4A < \(1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\) (1)
Đặt B = \(1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\)
3B = \(3-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{97}}-\frac{1}{3^{98}}\)
3B + B = \(\left(3-1+\frac{1}{3}-\frac{1}{3^2}+...+\frac{1}{3^{97}}-\frac{1}{3^{98}}\right)+\left(1-\frac{1}{3}+\frac{1}{3^2}-\frac{1}{3^3}+...+\frac{1}{3^{98}}-\frac{1}{3^{99}}\right)\)
4B = \(3-\frac{1}{3^{99}}\)
=> 4B < 3
=> B < \(\frac{3}{4}\) (2)
Từ (1) và (2) suy ra 4A < B < \(\frac{3}{4}\)=> A < \(\frac{3}{16}\)(đpcm)
Ta có 3A= \(^{3^2+3^3+3^4+...+3^{100}}\)
3A-A=2A= (\(3^2+3^3+3^4+...+3^{100}\))-(\(3+3^2+3^3+...+3^{99}\))
2A= \(3^{100}-3\)
theo bài ra ta có
2A+3=\(3^n\)= \(3^{100}-3+3=3^n\)=\(^{3^{100}}\)\(\Rightarrow\)n=100
bài 1.
a,vì /x/<=3 nên x thuộc{+1;+2;+3}
tổng là 0 vì tổng mỗi cặp số đối nhau bằng 0
vậy tổng là 0
tôi ko có thời gian chỉ trả lời phần a thoi phần b tương tự
Bài 1 : Ta có : \(A=3^{n+2}-2^{n+2}+3^n-2^n\)
\(=\left(3^{n+2}+3^n\right)-\left(2^{n+2}+2^n\right)\)
\(=3^n\left(9+1\right)-2^n\left(4+1\right)\)
\(=3^n.10-2^n.5\)
\(=3^n.10-2^{n-1}.10\)
\(=10\left(3^n-2^{n-1}\right)\)
\(=\overline{......0}\)
\(\Rightarrow\)Chữ số tận cùng của \(A\)là \(0\)
Bài 3:
a)Ta có : \(C=2+2^2+2^3+...+2^{99}+2^{100}\)
\(=\left(2+2^2+2^3+2^4\right)+\left(2^5+2^6+2^7+2^8\right)+...+\left(2^{97}+2^{98}+2^{99}+2^{100}\right)\)
\(=\left(2+2^2+2^3+2^4\right)+2^4\left(2+2^2+2^3+2^4\right)+...+2^{96}\left(2+2^2+2^3+2^4\right)\)
\(=31+2^4.31+...+2^{96}.31\)
\(=31\left(1+2^4+...+2^{96}\right)⋮31\)
\(\Rightarrow\)\(đpcm\)
b) Ta có : \(C=2+2^2+2^3+...+2^{99}+2^{100}\)
\(\Rightarrow2C=2^2+2^3+2^4+...+2^{100}+2^{101}\)
\(\Rightarrow2C-C=\left(2^2+2^3+2^4+...+2^{100}+2^{101}\right)-\left(2+2^2+2^3+...+2^{99}+2^{100}\right)\)
\(\Rightarrow C=2^{101}-2\)
Mà \(2^{2x}-2=C\)
\(\Rightarrow2^{2x}-2=2^{101}-2\)
\(\Rightarrow2^{2x}=2^{101}\)
\(\Rightarrow2x=101\)
\(\Rightarrow x=\frac{101}{2}\)
Vậy \(x=\frac{101}{2}\)
Bài 2:
Ta có : \(\overline{abcd}=1000a+100b+10c+d\)
\(=1000a+96b+8c+\left(d+2c+4b\right)\)
\(=8\left(125a+12b+c\right)+\left(d+2c+4b\right)\)
Vì \(\hept{\begin{cases}d+2c+4b⋮8\\8\left(125a+12b+c\right)⋮8\end{cases}}\)
\(\Rightarrow\overline{abcd}⋮8\)
\(\Rightarrowđpcm\)
1,
\(A=2^0+2^1+2^2+..+2^{2006}\)
\(=1+2+2^2+...+2^{2016}\)
\(2A=2+2^2+2^3+..+2^{2007}\)
\(2A-A=\left(2+2^2+2^3+..+2^{2007}\right)-\left(1+2+2^2+..+2^{2006}\right)\)
\(A=2^{2017}-1\)
\(B=1+3+3^2+..+3^{100}\)
\(3B=3+3^2+3^3+..+3^{101}\)
\(3B-B=\left(3+3^2+..+3^{101}\right)-\left(1+3+..+3^{100}\right)\)
\(2B=3^{101}-1\)
\(\Rightarrow B=\frac{3^{100}-1}{2}\)
\(D=1+5+5^2+...+5^{2000}\)
\(5D=5+5^2+5^3+...+5^{2001}\)
\(5D-D=\left(5+5^2+..+5^{2001}\right)-\left(1+5+...+5^{2000}\right)\)
\(4D=5^{2001}-1\)
\(D=\frac{5^{2001}-1}{4}\)
Bài 1: Ta có: \(B=3+3^2+3^3+...+3^{2005}\)
\(3B=3^2+3^3+3^4+...+3^{2006}\)
\(3A-A=3^{2006}-3\)
Hay \(2A=3^{2006}-3\)
+) Ta có: 2B+3=\(\left(3^{2006}-3\right)+3\)
\(\Rightarrow2B+3=3^{2006}\)
Vậy 2B+3 là lũy thừa của 3
b) Ta có: \(A=3+3^2+...+3^{100}\)
\(3A=3^2+3^3+...+3^{101}\)
\(3A-A=3^{101}-3\)
Hay \(2A=3^{101}-3\)
+) theo đề ra, ta có: \(2A+3=3^n\)
\(\Rightarrow\left(3^{101}-3\right)+3=3^{101}=3^n\)
\(\Rightarrow n=101\)
Mỏi tay wóa!!! Học tốt nha^^
B1
Có B=3+32+...+32005
=>3B=32+33+...+32006
=>2B=3B-B=32006-3
=>2B+3=32006-3+3=32006
=>Đpcm
B2
Có A=3+32+..+3100
=>3A=32+33+...+3101
=>2A=3A-A=3101-3
=>2A+3=3101-3+3=3101=3n
=>n=101