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A = 1 - 2 + 3 - 4 + 5 - 6 + ...... + 19 - 20
= (1 - 2) + (3 - 4) +( 5 - 6) + ...... + (19 - 20)
= -1 + (-1) + (-1) + ...... + (-1)
= -1 x 10
= -10
A=1-2+3-4+5-6+.....+19-20
A=(1-2)+(3-4)+(5-6)+....+(19-20)
A=(-1)+(-1)+(-1)+...+(-1) co 10 so -1
=>A= (-1)×10=-10
Ma -10 chia het cho 2 va 5 nhung khong chia het cho 3
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
Nếu $x\geq 2$ thì:
$x-2-5(x-4)=20-5x$
$x-2=20-5x+5(x-4)=0$
$\Rightarrow x=2$ (tm)
Nếu $x<2$ thì:
$2-x-5(x-4)=20-5x$
$2-x=20-5x+5(x-4)=0$
$\Rightarrow x=2$ (loại do $x<2$)
Vậy $x=2$
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\(-2.\left(-x-5\right)+28=20-3.\left(x+4\right)\)
\(\Rightarrow2x+10+28=20-3x-12\)
\(\Rightarrow2x+3x=20-12-10-28\)
\(\Rightarrow5x=-30\)
\(\Rightarrow x=-6\)
Vậy x = -6
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20 . 2^x + 1 = 10.4^2 + 1
20 . 2^x + 1 = 10 . 16 + 1
20 . 2^x + 1 = 161
20 . 2^x = 161 - 1
20 . 2^x = 160
2^x = 8
2^x = 2^3
=> x = 3
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(-25)-(5.x-3)=4
5.x-3 = -25 - 4
5.x-3=-29
5.x=-26
x=-26/5
x=-5,2
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( |x| . 2 + 3).4-8 = 20
( |x| . 2 + 3).4 = 20 + 8
|x| . 2 + 3 = 28 : 4
|x| . 2 = 7 - 3
|x| . 2 = 4
|x| = 4 : 2
|x| = 2
\(\Rightarrow\) x = 2 hoặc -2
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tìm x biết:
1/3+2/5.(x-1) = 4
=> 2/5.(x-1) = 4 - 1/3 = 11/3
=> x-1 = 11/3 : 2/5 = 55/6
=> x= 55/6 + 1 = 61/6
1/2-1/3:3x0 = -5
?????? đề hay thật
\(20^{3x}=20^{13}\div\left(4.5\right)^4\)
\(20^{3x}=20^{13}\div20^4\)
\(20^{3x}=20^9\)
\(3x=9\)
\(x=3\)