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![](https://rs.olm.vn/images/avt/0.png?1311)
a) ( x - 3 ) x 5 = 21 Đáp án 7,2
b) 36 - 8 x x = 26 : đs 1,25
c) 3- 7,2 = 1,56 Đs ...
d) x / 6 x 4 = 1,248 ĐS 1.872
![](https://rs.olm.vn/images/avt/0.png?1311)
a,\(\Rightarrow x:3=1,56+7,2\)
\(\Rightarrow x:3=8,76\)
\(\Rightarrow x=8,76\cdot3\)
\(\Rightarrow x=26,28\)
b,\(\Rightarrow x:6=1,248:4\)
\(\Rightarrow x:6=0,312\)
\(\Rightarrow x=0,312\cdot6\)
\(\Rightarrow x=1,872\)
c,(x+x+x+x+...+x)+(2+5+8+....+29)=165
10 số hạng 10 số hạng
\(\Rightarrow\)10x+(29+2)*10:2=165
\(\Rightarrow\)10x+155=165
\(\Rightarrow10x=165-155\)
\(\Rightarrow10x=10\)
\(\Rightarrow x=10:10\)
\(\Rightarrow x=1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a) x : 6 × 4 = 1 , 248
x : 6 = 1 , 248 : 4
x : 6 = 0 , 312
x = 0 , 312 × 6
x = 1 , 872
b) x : 5 − 8 , 6 = 0 , 4
x : 5 = 0 , 4 + 8 , 6
x : 5 = 9
x = 9 × 5
x = 45
![](https://rs.olm.vn/images/avt/0.png?1311)
a) \(\dfrac{3}{4}\times x:\dfrac{1}{2}=\dfrac{4}{5}\)
\(\dfrac{3}{4}\times x=\dfrac{4}{5}\times\dfrac{1}{2}\)
\(\dfrac{3}{4}\times x=\dfrac{2}{5}\)
\(x=\dfrac{2}{5}:\dfrac{3}{4}\)
\(x=\dfrac{8}{15}\)
b) \(\left(14-x:6\right)\times7=84\)
\(14-x:6=84:7\)
\(14-x:6=12\)
\(x:6=14-12\)
\(x:6=2\)
\(x=6\times2=12\)
c) \(x\times1,5:2,4=0,3\)
\(x\times1,5=0,3\times2,4\)
\(x\times1,5=0,72\)
\(x=0,72:1,5\)
\(x=0,48\)
d) Thiếu x
![](https://rs.olm.vn/images/avt/0.png?1311)
Ta có : A=20/11×13 + 20/13×15 +20/15×17+...+20/53×55
A = 10 ×( 2/11×13+2/13×15+...12/53×55)
A = 10 ×(1/11-1/13+1/13-1/15+1/15-1/17+...+1/53-1/55)
A = 10 × (1/11-1/55)
A =10 × 4/55
A = 8/11
![](https://rs.olm.vn/images/avt/0.png?1311)
x*3,4=15,3
x = 15,3 : 3,4
x = 4,5
7,5:x=0,24
x = 7,5 : 0,24
x = 31,25
5/6:x=7/8
x = 5/6 : 7/8
x = 20/21
2/3*x=5/9
x = 5/9 : 2/3
x = 5/6
Xx3,4=15,3
X = 15,3 : 3,4
X =4.5
7,5:x=0,24
x=7,5 : 0,24
x= 31.25
5/6:x=7/8
x=5/6 : 7/8
x= 20/21
2/3xX=5/9
X= 5/9 : 2/3
X= 5/6
![](https://rs.olm.vn/images/avt/0.png?1311)
\(x-\dfrac{2}{3}.\left(x+9\right)=1\)
\(\Rightarrow x-\dfrac{2}{3}x-6=1\)
\(\Rightarrow\dfrac{1}{3}x=7\)
\(\Rightarrow x=21\)
X- 9 = 21:5
X- 9 = 4,2
x = 4,2 + 9
x = 13,2
X : 3 = 1,248 : 4
X : 3 = 0,312
x = 0,312 x 3
x = 0,936
x - 9 = 4,2
x = 4,2 + 9 =13,2
x : 3 = 0,312
x = 0,312 x 3 = 0,936