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NN
3
1 tháng 6 2019
\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{2019.2020}\)
\(A=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{2019}-\frac{1}{2020}\)
\(A=1-\frac{1}{2020}\)
\(A=\frac{2019}{2020}\)
1 tháng 6 2019
\(B=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{2017.2019}\)
\(2B=\frac{2}{1.3}+\frac{2}{3.5}=\frac{2}{5.7}+...+\frac{2}{2017.2019}\)
\(2B=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}=\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2017}-\frac{1}{2019}\)
\(2B=1-\frac{1}{2019}\)
\(2B=\frac{2018}{2019}\)
\(B=\frac{2018}{2019}:2=\frac{1009}{2019}\)
LQ
2
16 tháng 4 2018
a)\(5^{2x-3}-2\cdot5^2=5^2\cdot3\)
\(5^{2x-3}-2\cdot25=75\)
\(5^{2x-3}-50=75\)
\(5^{2x-3}=125\)
\(125=5^3\)
\(5^3=5^{\left(3+3\right):2}=5^3\Rightarrow x=3\)
Vậy \(x=3\)
b)\(\frac{2}{9}\cdot\left(5x+1\right):2-\frac{1}{18}=\frac{5}{36}\)
\(=\frac{2}{9}\cdot\left(5x+1\right):2=\frac{5}{36}+\frac{1}{18}\)
\(=\frac{2}{9}\cdot\left(5x+1\right)=\frac{7}{36}\cdot2\)
\(5x+1=\frac{7}{18}:\frac{2}{9}\)
\(x=\left(\frac{7}{4}-1\right):5=\frac{3}{20}\)
\(\Rightarrow x=\frac{3}{20}\)
6 tháng 6 2020
a, \(5^{2x-3}-2.5^2=5^2.3\)
\(\Leftrightarrow5^{2x-3}-2.25=25.3\)
\(\Leftrightarrow5^{2x-3}-50=75\)
\(\Leftrightarrow5^{2x-3}=125\)
\(\Leftrightarrow5^{2x-3}=5^3\)
\(\Leftrightarrow2x-3=3\)
\(\Leftrightarrow2x=6\)
\(\Leftrightarrow x=3\)
b, \(\frac{2}{9}.\left(5x+1\right):2-\frac{1}{18}=\frac{5}{36}\)
\(\Leftrightarrow\frac{2}{9}.\left(5x+1\right):2=\frac{7}{36}\)
\(\Leftrightarrow\frac{2}{9}.\left(5x+1\right)=\frac{7}{36}.2\)
\(\Leftrightarrow\frac{2}{9}.\left(5x+1\right)=\frac{7}{18}\)
\(\Leftrightarrow5x+1=\frac{7}{18}:\frac{2}{9}\)
\(\Leftrightarrow5x+1=\frac{7}{18}.\frac{9}{2}\)
\(\Leftrightarrow5x+1=\frac{7}{4}\)
\(\Leftrightarrow5x=\frac{3}{4}\)
\(\Leftrightarrow x=\frac{3}{4}:5=\frac{3}{4}.\frac{1}{5}=\frac{3}{20}\)
23 tháng 9 2017
a,(72000+18000)-(3x+3000)=12000
90000-(3x+3000)=12000
3x+3000=90000-12000
3x+3000=78000
3x=78000-3000
3x=75000
x=75000:3
x=25000
b,[3.(x+2):7]:4=120
3.(x+2):7=120:4
3.(x+2):7=30
3.(x+2)=30.7
3.(x+2)=210
x+2=210:3
x+2=70
x=70-2
x=68
16 tháng 3 2020
\(\text{a/96-3(x+1)=42}\)
\(3\left(x+1\right)=54\)
\(x+1=54:3\)
\(x+1=18\)
\(\Rightarrow x=17\)
\(\text{b/2.x-18=20}\)
\(2x=38\)
\(x=38:2\)
\(x=19\)
\(\text{c/134-5.(x+4)=34}\)
\(5\left(x+4\right)=100\)
\(x+4=100:5\)
\(x+4=20\)
\(\Rightarrow x=16\)
học tốt
16 tháng 3 2020
a) 96-3(x+1)=42
3(x+1)=96-42
3(x+1)=54
x+1=54:3
x+1=18
x=18-1
Vậy x=17
b) 2x-18=20
2x=20+18
2x=38
x=38:2
x=19
Vậy x=19.
c) 134-5(x+4)=34
5(x+4)=134-34
5(x+4)=100
x+4=100:5
x+4=20
x=20-4
x=16
Vậy x=16.
16 tháng 3 2020
a) 96-3(x+1)=42
3(x+1)=96-42
3(x+1)=54
x+1=54:3
x+1=18
x=18-1
x=17
Vậy x=17
b)2.x-18=20
2.x =20+18
2.x =38
x =38:2
x =19
Vậy x=19
c)134-5.(x+4)=34
5.(x+4)=134-34
5.(x+4)=100
x+4 =100:5
x+4 =20
x =20-4
x =16
Vậy x=16
Ta có:
\(\frac{2}{3}-\frac{1}{3}.\left(x-\frac{3}{2}\right)-\frac{1}{2}.\left(2x+1\right)=5\)
\(\Rightarrow\frac{2}{3}-\frac{1}{3}x+\frac{1}{3}.\frac{3}{2}-x-\frac{1}{2}=5\)
\(\Rightarrow\frac{2}{3}-\left(\frac{1}{3}x+x\right)+\frac{1}{2}-\frac{1}{2}=5\)
\(\Rightarrow\frac{4}{3}x=\frac{2}{3}-5=-\frac{13}{3}\)
\(\Rightarrow x=-\frac{13}{3}:\frac{4}{3}=-\frac{13}{3}.\frac{3}{4}=-\frac{13}{4}\)
Vậy x=-13/4