\(-\dfrac{10}{21}=\dfrac{5}{3}\cdot\dfrac{-2}{7}\)

\(\dfrac{2}{21}=\dfrac{2}{3}\cdot\dfrac{1}{7}\)

\(\dfrac{8}{15}=\dfrac{2}{3}\cdot\dfrac{4}{5}\)

a) \(-\dfrac{10}{21}=\dfrac{2}{3}.\dfrac{-5}{7}\)

b) \(\dfrac{2}{21}=\dfrac{2}{3}.\dfrac{1}{7}\)

c) \(\dfrac{8}{15}=\dfrac{2}{3}.\dfrac{4}{5}\)

Haizzz Dễ Thôi Chịu:)

a)\(\dfrac{2}{5}+\dfrac{3}{4}:x=\dfrac{-1}{2}\)

=>\(\dfrac{3}{4}:x=\dfrac{-1}{2}-\dfrac{2}{5}\)

=>\(\dfrac{3}{4}:x=\dfrac{-9}{10}\)

=>\(x=\dfrac{3}{4}:\dfrac{-9}{10}\)

=>\(x=\dfrac{3}{4}.\dfrac{-10}{9}\)

=>\(x=\dfrac{-5}{6}\)

b)\(\dfrac{5}{7}-\dfrac{2}{3}.x=\dfrac{4}{5}\)

=>\(\dfrac{2}{3}.x=\dfrac{5}{7}-\dfrac{4}{5}\)

=>\(\dfrac{2}{3}.x=\dfrac{-3}{35}\)

=>\(x=\dfrac{-3}{35}:\dfrac{2}{3}\)

=>\(x=\dfrac{-3}{35}.\dfrac{3}{2}\)

=>\(x=\dfrac{-9}{70}\)

c)\(\dfrac{1}{2}x+\dfrac{3}{5}x=\dfrac{-2}{3}\)

=>\(x\left(\dfrac{1}{2}+\dfrac{3}{5}\right)=\dfrac{-2}{3}\)

=>\(\dfrac{11}{10}x=\dfrac{-2}{3}\)

=>\(x=\dfrac{-2}{3}:\dfrac{11}{10}\)

=>\(x=\dfrac{-2}{3}.\dfrac{10}{11}\)

=>\(x=\dfrac{-20}{33}\)

d)\(\dfrac{4}{7}x-x=\dfrac{-9}{14}\)

=>\(x\left(\dfrac{4}{7}-1\right)=\dfrac{-9}{14}\)

=>\(\dfrac{-3}{7}x=\dfrac{-9}{14}\)

=>\(x=\dfrac{-9}{14}:\dfrac{-3}{7}\)

=>\(x=\dfrac{-9}{14}.\dfrac{-7}{3}\)
=>\(x=\dfrac{3}{2}\)

thi à

Câu 20:

Con gà nặng nhất

Câu 19: 

3,75=3+0,7+0,05

15,9:100<1,59

kiểm tra ko giúp nhé

Tham khảo:

- Nếu nối 2 cực với 2 đầu dây điện thì đèn sẽ sáng

- Chiều dòng điện khi đó là từ cực dương sang cực âm của pin

Câu 1: C

14C  10D  2B (câu 2 em ko chắc lắm ạ )

Ta có: \(C=\dfrac{\dfrac{2006}{2}+\dfrac{2006}{3}+\dfrac{2006}{4}+...+\dfrac{2006}{2007}}{\dfrac{2006}{1}+\dfrac{2005}{2}+\dfrac{2004}{3}+...+\dfrac{1}{2006}}\)

\(=\dfrac{\dfrac{2006}{2}+\dfrac{2006}{3}+\dfrac{2006}{4}+...+\dfrac{2006}{2007}}{1+\left(1+\dfrac{2005}{2}\right)+\left(1+\dfrac{2004}{3}\right)+...+\left(1+\dfrac{1}{2006}\right)}\)

\(=\dfrac{\dfrac{2006}{2}+\dfrac{2006}{3}+\dfrac{2006}{4}+...+\dfrac{2006}{2007}}{\dfrac{2007}{2007}+\dfrac{2007}{2}+\dfrac{2007}{3}+...+\dfrac{2007}{2006}}\)

\(=\dfrac{2006}{2007}\)

Bài 6: 

PTHH: \(Fe+2HCl\rightarrow FeCl_2+H_2\uparrow\)

              a_____2a______a_____a      (mol)

            \(2Al+6HCl\rightarrow2AlCl_3+3H_2\uparrow\)

               b______3b______b_____\(\dfrac{3}{2}\)b    (mol)

Ta lập được HPT: \(\left\{{}\begin{matrix}56a+27b=22,2\\a+\dfrac{3}{2}b=\dfrac{13,44}{22,4}=0,6\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a=0,3\\b=0,2\end{matrix}\right.\)

\(\Rightarrow\left\{{}\begin{matrix}\%m_{Fe}=\dfrac{0,3\cdot56}{22,2}\cdot100\%\approx75,68\%\\\%m_{Al}=24,32\%\end{matrix}\right.\)