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a) 305 - 5x = 290
5.(61-x) = 290
61-x = 58
x = 3
b) (3x - 24) .25 = 26
3x - 24 = 2
3x = 18
x=6
c) 8 + 3.(x-5)2 = 35
3.(x-5)2 = 27
(x-5)2 = 9 = 32 = (-3)2
=> x - 5 = 3 => x = 8
x-5 = - 3 => x = 2
KL:>.
d) 21 chia hết cho x - 2
\(\Rightarrow x-2\inƯ_{\left(21\right)}=\left\{\pm1;\pm3;\pm7;\pm21\right\}.\)
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rùi bn tự lập bảng xét giá trị nhé
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có:
A bằng 1+2+22+23+...+219
A bằng (1+2)+(22+23)+...+(218+219)
A bằng 1.(1+2)+22.(1+2)+...+218.(1+2)
A bằng 1.3 + 22.3 + ... + 218.3
A bằng 3.(1+22+...+218)
Suy ra A chia hết cho 3.
![](https://rs.olm.vn/images/avt/0.png?1311)
A= 1+3+3^2+...+3^100
3A=3x( 1+3+3^2+...+3^100 )
3A-A=(3+3^2+...+3^101)-( 1+3+3^2+...+3^100 )
2A=3^101-1
A= \(\frac{3^{101}-1}{2}\)
B= 1+3^2+3^4+...+3^100
\(3^2B\)= 3^2x( 1+3^2+3^4+...+3^100)
9B-B= (3^2+3^4+..+3^102)-( 1+3^2+3^4+...+3^100 )
8B= 3^102-1
B=\(\frac{3^{102}-1}{8}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
8 x 5 =40 bạn nhầm phải không,dù sao thì cũng cảm ơn nhe!
\(A=4+2^2+2^3+2^4+...+2^{20}\)
\(\Rightarrow A=2^2+2^2+2^3+2^4+...+2^{20}\)
\(\Rightarrow 2A=2^3+2^3+2^4+2^5+...+2^{21}\)
\(\Rightarrow2A-A=2^{21}+2^3-2^2-2^2\)
\(\Rightarrow A=2^2\left(2^{19}+2-1-1\right)\)
\(\Rightarrow A=2^{19}.2^2=2^{21}\)
\(\text{A}=4+2^2+2^3+2^4+...+2^{20}\)
\(\Rightarrow\text{A}=4+(2^2+2^3+...+2^{20})\)
\(\Rightarrow\text{A}-4=2^2+2^3+...+2^{20}\)
\(\Rightarrow2\text{(A}-4)=2^3+2^4+...+2^{21}\)
\(\text{A}-4=2(\text{A}-4)-(\text{A}-4)=(2^3+2^4+...+2^{21})-(2^2+2^3+...+2^{20})\)
\(\Rightarrow\text{A}-4=(2^3-2^3)+(2^4-2^4)+....+(2^{21}-2^2)\)
\(\Rightarrow\text{A}-4=2^{21}-4\)
\(\Rightarrow\text{A}=2^{21}-4+4\Rightarrow\text{A}=2^{21}\)
Chúc bạn hok tốt :>