x - [45 +20 ] : 21 =
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![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có \(VP=\frac{\left(20+29\right)\times10}{2}=245\)
=> 20xX+45=245
20xX=200
X=10
20.x + 45 = 20 + 21 + 22 + ... + 29
=> 20.x + 45 = (29 + 20).[(29 - 20) : 1 + 1] : 2
=> 20.x + 45 = 49 . 10 : 2
=> 20.x + 45 = 245
=> 20.x = 245 - 45
=> 20.x = 200
=> x = 200 : 20
=> x =10
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\frac{27}{20};\frac{45}{40};\frac{21}{20};\frac{20}{21}\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
a, Ta có: x ∈ B(5) = {0;5;10;15;20;25;...}
Mà x < 20 => x ∈ {0;5;10;15}
b, Ta có: x ⋮ 24; x ⋮ 45 => x ∈ BC{24;45}
24 = 2 3 . 3 ; 45 = 3 2 . 5 => BCNN(24;45) = 2 3 . 3 2 . 5 = 360
=> BC(24;45) = B(360) = {0;360;720;...}
Mà 200 < x < 500 => x = 360
c, Ta có:
452 = px+32 => 420 ⋮ x
321 = qx+21 => 320 ⋮ x
=> x ∈ ƯC(420;320)
420 = 2 2 . 3 . 5 . 7
320 = 2 6 . 5
=> ƯCLN(420;320) = 20 = 2 2 . 5
=> ƯC(420;320) = Ư(20) = {1;2;4;5;10;20}
Vậy x ∈ {1;2;4;5;10;20}
![](https://rs.olm.vn/images/avt/0.png?1311)
112 - (x+4)=1339:13
112 - (x+4)=103
x+4= 112-103
x+4=9
x=9-4
x=5
_________
468+85xX=2508
85 x X = 2508 - 468
85 x X = 2040
X= 2040 : 85
X=24
____
Xx2,5=2,5x4
X x 2,5 = 10
X= 10:2,5
X=4
________
x-270:45=120
X - 6 = 120
X= 120+6
X=126
575-(6xX)+70=445
(6 x X) + 70 = 575 - 445
(6 x X) + 70= 130
6 x X = 130 - 70
6 x X = 60
X= 60:6
X=10
______
x-105:21=15
X - 5 = 15
X= 15+5
X=20
______
3200:(405-x)=8
405 - x = 3200 : 8
405 - x = 400
x= 405 - 400
x= 5
_______
43575-75xX=42450
75 x X = 43575 - 42450
75 x X = 1125
X= 1125:75
X= 15
______
8,75xX+1,25xX=20
(8,75 + 1,25) x X = 20
10 x X = 20
X=20:10
X=2
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a.7-\left(x+15\right)=21-2x\)
\(\Leftrightarrow7-x-15=21-2x\)
\(\Leftrightarrow7-15=21-2x+x\)
\(\Leftrightarrow-8=21-x\)
\(\Leftrightarrow x=21+8\)
\(\Leftrightarrow x=29\)
\(\text{Vậy }x=29\)
\(b.-20-\left(30-x\right)=45\)
\(\Leftrightarrow30-x=-20-45\)
\(\Leftrightarrow30-x=-65\)
\(\Leftrightarrow x=30+65\)
\(\Leftrightarrow x=95\)
\(\text{Vậy }x=95\)
\(a,7-\left(x+15\right)=21-2x\)
\(7-x-15=21-2x\)
\(-x+2x=21-7+15\)
\(x=29\)
\(b,-20-\left(30-x\right)=45\)
\(-20-30+x=45\)
\(x=45+30+20\)
\(x=95\)
Nếu toán lp 3 thì thiếu đề r pn!
Bn ra đề thiếu rồi!!