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1.4x - 5(-3+x)=7
4x - 5(x-3) =7
4x - 5x + 15=7
-1x + 15=7
-1x =-8
=> x =8
2.5(x-3) - 2(x+6)=9
5x - 15 -2x -12=9
5x - 2x -15 - 12=9
5x - 2x=9 + 12 + 15
5x - 2x= 36
3x = 36
=> x = 12
3.4(x-1) - 3(x-2)=15
4x - 4 - 3x + 6=15
4x - 3x =15 - 6 + 4
4x - 3x = 13
=> x = 13
Nhớ mink nhoa pn
(2011x2012+2012x2013)x(1+\(\frac{1}{2}:1\frac{1}{2}-1\frac{1}{3}\))
= A x(1+\(\frac{1}{3}-1\frac{1}{3}\))
=A x(\(\frac{4}{3}-1\frac{1}{3}\))
= A x 0
=0
A=1+2+3+...+2018=(1+2018)+(2+2017)+...(1009+1010)=2019x1009=2037171
B=(1+2019)+(3+2017)+...+(1009+1011)=2020x505=1020100
C=(2020+2)+(2018+4)+...+(1010+1012)=2022x505=1021110
a) \(\frac{113}{13}-\frac{24}{7}-\frac{53}{13}=\frac{60}{13}-\frac{24}{7}\)
=\(\frac{420-312}{91}=\frac{108}{91}\)
b) \(\frac{2}{7}.\left(\frac{51}{4}-\frac{31}{4}\right)=\frac{2}{7}.5=\frac{10}{7}\)
c) \(\frac{3}{5}+1-\frac{1}{20}\)= \(\frac{12}{20}+\frac{20}{20}-\frac{1}{20}=\frac{31}{20}\)
3/7+1/2- -3/70 =34/35
3/5 + -1/25 -35/100 =21/100
5/12-3/-16 + 3/4 =65/48
Bài 2:
Ta có: \(16x+40=10\cdot3^2+5\left(1+2+3\right)\)
\(\Leftrightarrow16x+40=90+30\)
\(\Leftrightarrow16x=80\)
hay x=5
=>S=2(1-1/3+1/3-1/4+....................-1/2020)
=>S=2*(1-1/2020)
=>s=2* 2019/2020
=>S=2019/1010
\(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+...+\frac{2}{2017\cdot2019}+\frac{2}{2019\cdot2021}\)
\(=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2017}-\frac{1}{2019}+\frac{1}{2019}-\frac{1}{2021}\)
\(=1-\frac{1}{2021}=\frac{2020}{2021}\)