a: Theo đề, ta có hệ phương trình:

\(\left\{{}\begin{matrix}-a+b=-20\\3a+b=8\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=7\\b=8-3a=8-3\cdot7=-13\end{matrix}\right.\)

18: \(\left(2,8x-32\right):\frac23=-90\)

=>\(\left(2,8x-32\right)=-90\cdot\frac23=-60\)

=>2,8x=-60+32=-28

=>x=-10

19: \(6\frac29\cdot x-1\frac38:\left(\frac38+\frac{9}{20}\right)=4\frac16\)

=>\(\frac{56}{9}x-\frac{11}{8}:\left(\frac{15}{40}+\frac{18}{40}\right)=\frac{25}{6}\)

=>\(\frac{56}{9}x-\frac{11}{8}:\frac{33}{40}=\frac{25}{6}\)

=>\(\frac{56}{9}x=\frac{25}{6}+\frac{11}{8}\cdot\frac{40}{33}=\frac{25}{6}+\frac53=\frac{25}{6}+\frac{10}{6}=\frac{35}{6}\)

=>\(x=\frac{35}{6}:\frac{56}{9}=\frac{35}{6}\cdot\frac{9}{56}=\frac32\cdot\frac58=\frac{15}{16}\)

20: \(\frac13+\frac16+\frac{1}{10}+\cdots+\frac{1}{\frac{x\left(x+1\right)}{2}}=\frac{2009}{2021}\)

=>\(\frac26+\frac{2}{12}+\frac{2}{20}+\cdots+\frac{2}{x\left(x+1\right)}=\frac{2009}{2021}\)

=>\(2\left(\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\cdots+\frac{1}{x\left(x+1\right)}\right)=\frac{2009}{2021}\)

=>\(2\left(\frac12-\frac13+\frac13-\frac14+\cdots+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{2009}{2021}\)

=>\(2\left(\frac12-\frac{1}{x+1}\right)=\frac{2009}{2021}\)

=>\(1-\frac{2}{x+1}=\frac{2009}{2021}\)

=>\(\frac{2}{x+1}=\frac{12}{2021}\)

=>\(x+1=2021\cdot\frac{2}{12}=\frac{2021}{6}\)

=>\(x=\frac{2021}{6}-1=\frac{2015}{6}\)

D nhé 

1. A

2. C

3. B

4. A

1) A

2) C

3) B

4) A

Câu nào bạn ơi

Câu 19,20