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i) ( 12- 3x ) : 20 = 0
=> 12 - 3x = 0
3x = 12
x=12 : 3 =3
k) ( 2x - 86 ) : 9999 = 0
=> 2x - 86 = 0
=> 2x = 86
=> x = 86 : 2 = 43
l) 83 : ( 5x + 28 ) = 1
5x + 28 = 83 : 1 = 83
5x = 83 - 28
5x =55
x=55 : 5 = 11
m) 98 : ( 72 - x ) = 98
72-x = 98 : 98
72 - x = 1
x = 72 - 1 = 71
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\(a,\Leftrightarrow x+9=39\Leftrightarrow x=30\\ b,\Leftrightarrow73-x=69\Leftrightarrow x=4\\ c,\Leftrightarrow x+7=38\Leftrightarrow x=31\\ d,\Leftrightarrow x+4=178\Leftrightarrow x=174\\ e,\Leftrightarrow2\left(x-51\right)=16+20=36\Leftrightarrow x-51=18\Leftrightarrow x=69\\ f,\Leftrightarrow x-19=9\Leftrightarrow x=28\\ g,\Leftrightarrow4\left(x-3\right)=49-1=48\Leftrightarrow x-3=12\Leftrightarrow x=15\)
a) 45 - ( x + 9 ) = 6
45 - x = 6 + 9
45 - x = 15
x = 45 - 15
x = 30
b) 89 - ( 73 - x ) = 20
89 - x = 20 + 73
89 - x = 93
x = 93 - 89
x = 4
c) 198 - ( x + 4 ) = 120
198 - x = 120 + 4
198 - x = 124
x = 198 - 124
x = 74
d) ( x + 7 ) - 25 = 13
x - 25 = 13 + 7
x - 25 = 20
x = 20 + 25
x = 45
e) 2 ( x - 51 ) = 2.23 + 20
2 ( x - 51 ) = 64 + 20
2 ( x - 51 ) = 84
x - 51 = 84 - 2
x - 51 = 82
x = 82 + 51
x = 133
f) 450 : ( x - 19 ) = 50
450 : x = 50 - 19
450 : x = 41
x = 450 : 41
x = 24
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a) \(\frac{5}{6}+\frac{11}{12}+\frac{19}{20}+\frac{29}{30}+\frac{41}{42}+\frac{55}{56}+\frac{71}{72}+\frac{89}{90}\)
\(=1-\frac{1}{6}+1-\frac{1}{12}+1-\frac{1}{20}+1-\frac{1}{30}+1-\frac{1}{42}+1-\frac{1}{56}+1-\frac{1}{72}+1-\frac{1}{90}\)
\(=8-\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\right)\)
\(=8-\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
\(=8-\left(\frac{3-2}{2.3}+\frac{4-3}{3.4}+\frac{5-4}{4.5}+\frac{6-5}{5.6}+\frac{7-6}{6.7}+\frac{8-7}{7.8}+\frac{9-8}{8.9}+\frac{10-9}{9.10}\right)\)
\(=8-\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)
\(=8-\left(\frac{1}{2}-\frac{1}{10}\right)=7,6\)
b) Bạn làm tương tự.
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=\dfrac{1}{2}+\dfrac{5}{6}+\dfrac{11}{12}+\dfrac{29}{30}+\dfrac{41}{42}+\dfrac{55}{56}+\dfrac{71}{72}+\dfrac{89}{90}\) (sửa đề)
\(=\left(1-\dfrac{1}{2}\right)+\left(1-\dfrac{1}{6}\right)+\left(1-\dfrac{1}{12}\right)+\left(1-\dfrac{1}{30}\right)+\left(1-\dfrac{1}{42}\right)+\left(1-\dfrac{1}{56}\right)+\left(1-\dfrac{1}{72}\right)+\left(1-\dfrac{1}{90}\right)\)
\(=\left(1+1+1...+1\right)-\left(\dfrac{1}{2}+\dfrac{1}{6}+\dfrac{1}{12}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\right)\)
\(=8-\left(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+\dfrac{1}{3\cdot4}+...+\dfrac{1}{9\cdot10}\right)\) ( có 8 số hạng 1)
\(=8-\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)
\(=8-\left(1-\dfrac{1}{10}\right)\)
\(=8-\dfrac{9}{10}\)
\(=\dfrac{80}{10}-\dfrac{9}{10}=\dfrac{71}{10}\)
A=1/2+5/6+11/12+19/20+29/30+41/42+55/56+71/72+89/90
=1−1/2+1−1/6+1−1/12+1−1/20+1−1/30+1−1/42+1−1/56+1−1/72+1−1/90
=9−(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90)
=9−(1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10)
=9-(1-1/2+1/2-1/3+.....+1/9-1/10)
=9−(1−1/10)
=9−1+1/10=8+1/10=81/10
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1/ Thay x=2020 vào biểu thức ta có:
2020+18-2020-22
=(2020-2020)+(18-22)
=0+(-4)
=0-4
=-4
2/Thay x=-198;a=9999 vào biểu thức ta có:
-(-198)-9999+12+9999
=198-9999+12+9999
=(198+12)-(9999-9999)
=210-0
=210
3/Thay a=10;m=-12345 vào biểu thức ta có:
10-(-12345)+7-8+(-12345)
=10+12345+7-8-12345
=(10+7-8)+(12345-12345)
=9+0
=9
4/Thay x=2019;m=72 vào biểu thức ta có:
72-24-2019+24+2019
=72-(24-24)-(2019-2019)
=72-0-0
=72
5/Thay y=-2021 vào biểu thức ta có:
(-80)-(-2021+10)+90
=-80+2021+10+90
=-80+2021+(10+90)
=-80+2021+100
=(100-80)+2021
=20+2021
=2041
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1/ \(x+18-x-22\\ =x-x+18-22\\ =-4\)
2/ \(-x-a+12+a\\ =-x-a+a+12\\ =-x+12\)
với x = -198
=> \(-x+12\\ =-198+12=-186\)
3/ \(a-m+7-8+m\\ =a-m+m-1\\ =a-1\)
với a = 10
=> a - 1
= 10 - 1 = 9
4/ \(m-24-x+24+x\\ =m-x+x-24+24\\ =m\)
mà m = 72 => biểu thức có giá trị là 72
5/ \(\left(-80\right)-\left(y+10\right)+90\\ =\left[\left(-80\right)+90\right]-y-10\\ =10-10-y\\ =-y\)
mà y = -2021 nên biểu thức có giá trị là 2021
9999=x+89-72
x+89-72=9999
x= 9999+72-89
x =9982
Vậy x = 9982
9999 = \(x\) + 89 - 72
\(x\) + 17 = 9999
\(x\) = 9999 - 17
\(x\) = 9982