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\(\left(x-7\right)\left(x+2019\right)=0\)
\(\Rightarrow\orbr{\begin{cases}x-7=0\\x+2019=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=7\\x=-2019\end{cases}}\)
\(9-25=\left(7-x\right)-\left(25+7\right)\)
\(\Leftrightarrow-16=7-x-25-7\)
\(\Leftrightarrow-x=-16+25\)
\(\Leftrightarrow-x=9\)
\(\Leftrightarrow x=-9\)
\(2\left(4x-2x\right)-7x=15\)
\(\Leftrightarrow4x-7x=15\)
\(\Leftrightarrow x=-5\)
a ) 9 - 25 = ( 7 - x ) - ( 25 + 7 )
9 - 25 = 7 - x - 25 - 7
9 - 25 - 7 + 25 + 7 = -x
9 = - x
=> x = -9
Vậy x = -9
b) 2 . ( 4x - 2x ) - 7x = 15
8x - 4x - 7x = 15
-3x = 15
x = 15 : ( - 3 )
x = -5
Vậy x = -5
c ) ( x - 7 ). ( x + 2019 ) = 0
=> x - 7 = 0 hoặc x + 2019 = 0
=> x = 7 hoặc x = - 2019
vậy x \(\in\){ 7 ; -2019 }
![](https://rs.olm.vn/images/avt/0.png?1311)
9-25=(7-x)-(25+7)
( 7 - x ) - ( 25 + 7 ) = 9 - 25
( 7 - x ) - 32 = - 16
7 - x = ( - 16 ) + 32
7 - x = 16
x = 7 - 16
x = - 9
Vậy x = - 9
9 - 25 = ( 7 - x ) - ( 25 + 7 )
x = 7 - 25 - 7 - 9 + 25
x = ( 7 - 7 ) + ( 25 - 25 ) -9
x = 0 + 0 - 9
x= - 9
=> x= - 9
Vậy x = - 9
![](https://rs.olm.vn/images/avt/0.png?1311)
a.-12 ( x-5 ) +7 (3-x) = 5
-12x +60 + 21 - 7x =5
60+21-5 = 12x + 7x
76 = 19x
x = 4
b.30(x +2) - 6(x-5) -24x = 100
30x+60 - 6x -30-24x = 100
60-30-100 = 30x -6x - 24x
-70 = 0x
x = -70
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(1-\frac{1}{2}\right)\left(1-\frac{1}{3}\right)\left(1-\frac{1}{4}\right).....\left(1-\frac{1}{10}\right)\)
\(=\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.......\frac{9}{10}=\frac{1.2.3......9}{2.3.4.....10}=\frac{1}{10}\)
Bài 1:Tính nhanh
a,149+152+257+138 = 149 + 257 + ( 152 + 138 ) = 696
b,12.19.25 = ( 12.25 ).19 = 5700
c,79.37+79.18-55.78
= 79.(37+18) -55.78
= 79.55-55.78
=55.(79-78)
=55
Bài 2:Tìm x biết:
a,83+7.(25-x)=167
7. (25-x)=167-83
7. (25-x)=84
25-x=84:7
25-x=12
x=13
b,149-9.(x+12)=14
9.(x+12)=149-14
9.(x+12)=135
x+12= 135:9
x+12=15
x = 3
c,(x+9). (x+7)=143
\(\Leftrightarrow x+9=143\) hoặc \(x+7=143\)
\(\Leftrightarrow x=134\) hoặc \(x=136\)
Bài 1:Tính nhanh
a,149+152+257+138
= (152 + 138) + (149 + 257)
= 290 + 406 = 696
b,12.19.25
= (25.12) .19
= 300.19 = 5700
c,79.37 + 79.18 - 55.78
= 79.(37 + 18) - 55.78
= 79.55 - 55.78
= 55.(79 - 78)
= 55.1 = 55
Bài 2:Tìm x biết:
a,83 + 7.(25 - x) = 167
7.(25 - x) = 167 - 83
7.(25 - x) = 84
25 - x = 84 : 7
25 - x = 12
x = 25 - 12 = 13
b,149 - 9.(x + 12) = 14
9.(x + 12) = 149 - 14
9.(x + 12) = 135
x + 12 = 135 : 9
x + 12 = 15
x = 15 - 12 = 3
c,(x + 9).(x + 7) = 143
TH1: (x + 9) = 11 => x = 2
(x + 7) = 13 => x = 6
TH2: (x + 9) = 13 => x = 4
(x + 7) = 11 => x = 4
TH3: (x + 9) = -11 => x = -20
(x + 7) = -13 => x = -20
TH4: (x + 9) = -13 => x = -22
(x + 7) = -11 => x = -18