a. 7/ 2 - 1,5 + -8/11
b. [ 1 và 3/7 + 3 và 4/7] ; 6/5 - 2.5 . 8/15
c. 7/9 . 35/17-7/9.25/17-7/9
d. [120% + 1 và 3/5 ] ; 27/5 - 4,5 .4/9
e. -5/7 . x = 25/12
g. 7/11+1/11 .x = 0,3 - 3 và 2/5
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2 giờ 20 phút = 2\(\dfrac{1}{3}\) giờ
hoặc 2 giờ 20 phút = \(\dfrac{7}{3}\) giờ
a: \(\dfrac{1}{1\cdot2}+\dfrac{1}{2\cdot3}+...+\dfrac{1}{x\left(x+1\right)}=\dfrac{2021}{2022}\)
=>\(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{x}-\dfrac{1}{x+1}=\dfrac{2021}{2022}\)
=>\(1-\dfrac{1}{x+1}=\dfrac{2021}{2022}\)
=>\(\dfrac{1}{x+1}=1-\dfrac{2021}{2022}=\dfrac{1}{2022}\)
=>x+1=2022
=>x=2021
b: Sửa đề: \(\dfrac{x+1}{99}+\dfrac{x+2}{98}+\dfrac{x+3}{97}+\dfrac{x+4}{96}=-4\)
=>\(\left(\dfrac{x+1}{99}+1\right)+\left(\dfrac{x+2}{98}+1\right)+\left(\dfrac{x+3}{97}+1\right)+\left(\dfrac{x+4}{96}+1\right)=0\)
=>\(\dfrac{x+100}{99}+\dfrac{x+100}{98}+\dfrac{x+100}{97}+\dfrac{x+100}{96}=0\)
=>\(\left(x+100\right)\left(\dfrac{1}{99}+\dfrac{1}{98}+\dfrac{1}{97}+\dfrac{1}{96}\right)=0\)
=>x+100=0
=>x=-100
\(\dfrac{20\times6\times2}{10\times16\times24}=\dfrac{6\times2}{24}\times\dfrac{20}{10}\times\dfrac{1}{16}\)
\(=\dfrac{1}{2}\times2\times\dfrac{1}{16}=\dfrac{1}{16}\)
\(189:\left[628-\left(2x-1\right)^2\right]=3^2\cdot7\)
=>\(628-\left(2x-1\right)^2=\dfrac{189}{63}=3\)
=>\(\left(2x-1\right)^2=628-3=625\)
=>\(\left[{}\begin{matrix}2x-1=25\\2x-1=-25\end{matrix}\right.\)
=>\(\left[{}\begin{matrix}2x=26\\2x=-24\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=13\\x=-12\end{matrix}\right.\)
\(189:\left[628-\left(2x-1\right)^2\right]=3^2.7\)
\(\Rightarrow628-\left(2x-1\right)^2=\dfrac{189}{63}\)
\(\Rightarrow628-\left(2x-1\right)^2=3\)
\(\Rightarrow\left(2x-1\right)^2=625\)
\(\Rightarrow\left[{}\begin{matrix}2x-1=25\\2x-1=-25\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=13\\x=-12\end{matrix}\right.\)
Vận tốc ô tô:
\(135:2,25=60\)(km/h)
Vận tốc xe máy:
\(60\times\dfrac{2}{3}=40\)(km/h)
Đáp số: 40 km/h
Vận tốc của ô tô là
\(135\div2,25=60\left(km/h\right)\)
Vận tốc của xe máy là
\(60\times\dfrac{2}{3}=40\left(km/h\right)\)
\(\dfrac{4}{5}\times\dfrac{3}{7}+\dfrac{4}{5}\times\dfrac{6}{7}-\dfrac{4}{5}\times\dfrac{4}{14}\)
\(=\dfrac{4}{5}\times\left(\dfrac{3}{7}+\dfrac{6}{7}-\dfrac{4}{14}\right)=\dfrac{4}{5}\times1=\dfrac{4}{5}\)
a: \(\dfrac{7}{2}-1,5+\dfrac{-8}{11}=3,5-1,5-\dfrac{8}{11}=2-\dfrac{8}{11}=\dfrac{22-8}{11}=\dfrac{14}{11}\)
b: \(\left(1\dfrac{3}{7}+3\dfrac{4}{7}\right):\dfrac{6}{5}-2,5\cdot\dfrac{8}{15}\)
\(=\left(1+\dfrac{3}{7}+3+\dfrac{4}{7}\right)\cdot\dfrac{5}{6}-\dfrac{5}{2}\cdot\dfrac{8}{15}\)
\(=5\cdot\dfrac{5}{6}-\dfrac{40}{30}=\dfrac{25}{6}-\dfrac{4}{3}=\dfrac{17}{6}\)
c: \(\dfrac{7}{9}\cdot\dfrac{35}{17}-\dfrac{7}{9}\cdot\dfrac{25}{17}-\dfrac{7}{9}\)
\(=\dfrac{7}{9}\left(\dfrac{35}{17}-\dfrac{25}{17}-1\right)\)
\(=\dfrac{7}{9}\cdot\dfrac{-7}{17}=\dfrac{-49}{117}\)
d: \(\left(120\%+1\dfrac{3}{5}\right):\dfrac{27}{5}-4,5\cdot\dfrac{4}{9}\)
\(=\left(1,2+1,6\right)\cdot\dfrac{5}{27}-2\)
\(=\dfrac{14}{27}-2=\dfrac{14}{27}-\dfrac{54}{27}=-\dfrac{40}{27}\)
e: \(\dfrac{-5}{7}\cdot x=\dfrac{25}{12}\)
=>\(x=-\dfrac{25}{12}:\dfrac{5}{7}=-\dfrac{25}{12}\cdot\dfrac{7}{5}=\dfrac{-5\cdot7}{12}=-\dfrac{35}{12}\)
g: \(\dfrac{7}{11}+\dfrac{1}{11}\cdot x=0,3-3\dfrac{2}{5}\)
=>\(\dfrac{x}{11}=0,3-3,4-\dfrac{7}{11}=-\dfrac{411}{110}\)
=>\(x=-\dfrac{411}{110}\cdot11=-\dfrac{411}{10}\)
hơi dài đó.