Gọi số cây của ba lớp 7A ; 7B ; 7C lần lượt là a ; b và c ( cây ) ( a , b , c ∈ N* )

Theo bài ra , ta có :

b + c - a = 15

\(\frac{a}{6}=\frac{b}{4}=\frac{c}{5}\)

Áp dụng tính chất dãy tỉ số bằng nhau , ta có :

\(\frac{a}{6}=\frac{b}{4}=\frac{c}{5}=\frac{b+c-a}{4+5-6}=\frac{15}{3}=5\)

\(\Rightarrow\hept{\begin{cases}a=5.6=30\\b=5.4=20\\c=5.5=25\end{cases}}\)

ta có: \(\frac{11}{12}.\left(\frac{19}{41}+\frac{22}{41}\right)\)

= \(\frac{11}{12}\)

\(\frac{11}{12}.\frac{19}{41}+\frac{11}{12}.\frac{22}{41}\)

\(=\frac{11}{12}.\left(\frac{19}{41}+\frac{22}{41}\right)\)

\(=\frac{11}{12}.1=\frac{11}{12}\)

\(15\frac{1}{4}:\left(-\frac{5}{7}\right)-25\frac{1}{4}:\left(-\frac{5}{7}\right)\)

\(=15\frac{1}{4}.\left(-\frac{7}{5}\right)-25\frac{1}{4}:\left(-\frac{7}{5}\right)\)

\(=-\frac{7}{5}\left(15\frac{1}{4}-25\frac{1}{4}\right)\)

\(=-\frac{7}{5}.\left(-10\right)=14\)

Ta có: \(9.\left(\frac{-1}{27}\right)+\frac{1}{3}\)

\(=\left(\frac{-1}{3}\right)+\frac{1}{3}\)

= 0

9 x  ( -1/3 ) ^3 + 1/3 

= 9 x ( -1/27 ) + 1/3 

= - 1/3 + 1/3 

= 0 

\(\frac{3}{7}.19\frac{1}{3}-\frac{3}{7}.33\frac{1}{3}\)

= \(\frac{3}{7}.\left(19\frac{1}{3}-33\frac{1}{3}\right)\)

=\(\frac{3}{7}.-14\)

=\(-6\)

ĐK: \(x\ge3\).

\(A=\left(x-1\right)^2+\sqrt{x-3}-5\ge\left(3-1\right)^2+\sqrt{3-3}-5=-1\)

Dấu \(=\)khi \(x=3\).

Anh ơi, anh có thể giải thích đc ra không ạ, em vẫn chưa hiểu lắm ạ

\(\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2021}\right)x=\frac{2020}{1}+\frac{2019}{2}+...+\frac{1}{2020}\)

\(\Leftrightarrow\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2021}\right)x=1+\left(\frac{2019}{2}+1\right)+...+\left(\frac{1}{2020}+1\right)\)

\(\Leftrightarrow\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2021}\right)x=\frac{2021}{2021}+\frac{2021}{2}+\frac{2021}{3}+...+\frac{2021}{2020}\)

\(\Leftrightarrow\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2021}\right)x=2021\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{2021}\right)\)

\(\Leftrightarrow x=2021\)

MÌNH CẦN GẤP

a, - \(\dfrac{11}{25}\) = \(\dfrac{-6}{25}\) + \(\dfrac{-5}{25}\) + 

    - \(\dfrac{11}{25}\) =  \(\dfrac{-1}{25}\) + \(\dfrac{-10}{25}\) + 

    - \(\dfrac{11}{25}\) = \(\dfrac{-3}{25}\) + \(\dfrac{-8}{25}\) +  

b, - \(\dfrac{11}{25}\) = \(\dfrac{6}{25}\) - \(\dfrac{17}{25}\)

   - \(\dfrac{11}{25}\) = \(\dfrac{7}{25}\) - \(\dfrac{18}{25}\)

  - \(\dfrac{11}{25}\)  = \(\dfrac{8}{25}\) - \(\dfrac{19}{25}\)