a) a/b = 7/8 - 5/6

a/b = 1/24

b) a/b = 3/4 : 2/3

a/b = 9/8

c) a/b = 5/8 - 5/12

a/b= 5/24

d) a/b = 3/8 - 5/8

a/b = -2/8 -1/4

a) \(\dfrac{x}{-6}=\dfrac{-15}{45}\)

    \(\dfrac{-x}{6}=\dfrac{-15}{45}\)

    \(\dfrac{x}{6}=\dfrac{15}{45}\)

    \(x=\dfrac{\left(15\cdot6\right)}{45}\)

    \(x=2\)

b) \(\dfrac{x}{5}=\dfrac{16}{25}\)

    \(x=\dfrac{\left(16\cdot5\right)}{25}\)

    \(x=\dfrac{80}{25}\)

   \(x=\dfrac{16}{5}\)

c) \(\dfrac{5}{x-3}=\dfrac{20}{-12}\)

  \(x-3=\dfrac{\left(5\cdot-12\right)}{20}\)

 \(x-3=-3\)

 \(x=\left(-3\right)+3\)

\(x=0\)

d) \(\dfrac{2}{5}\cdot x=\dfrac{6}{35}\)

         \(x=\dfrac{6}{35}\div\dfrac{2}{5}\)

        \(x=\dfrac{3}{7}\)

\(\left(x+5\right)^2-\left(x-3\right)\left(x+7\right)=19\\ x^2+10x+25-\left(x^2+7x-3x-21\right)=19\\ x^2+10x+25-x^2-7x+3x+21=19\\ 6x+46=19\\ 6x=19-46\\ 6x=-27\\ x=\dfrac{-27}{6}\\ x=-4,5\)

Chọn D

D

Chọn đáp án: B. Các câu 1 – 2 – 4 – 6 – 8

Chọn C

a: \(x-\frac25+\frac{7}{12}=\frac12\cdot\frac34\cdot\frac{-9}{16}\)

=>\(x-\frac{24}{60}+\frac{35}{60}=\frac38\cdot\frac{-9}{16}=\frac{-27}{128}\)

=>\(x=-\frac{27}{128}-\frac{11}{60}=\frac{-27\cdot15-11\cdot32}{1920}=-\frac{757}{1920}\)

b: \(\frac49x-\frac12=-\frac59\)

=>\(\frac49x=-\frac59+\frac12=\frac{-10+9}{18}=\frac{-1}{18}\)

=>\(x=-\frac{1}{18}:\frac49=-\frac{1}{18}\cdot\frac94=\frac{-1}{2\cdot4}=-\frac18\)

c: \(-\frac23x+\frac58=\frac{7}{12}\)

=>\(-\frac23x=\frac{7}{12}-\frac58=\frac{14}{24}-\frac{15}{24}=-\frac{1}{24}\)

=>\(x=\frac{1}{24}:\frac23=\frac{1}{24}\cdot\frac32=\frac{1}{8\cdot2}=\frac{1}{16}\)

d: \(\frac25x:\frac73\cdot\frac18=\frac{29}{15}\)

=>\(x\cdot\frac25\cdot\frac37\cdot\frac18=\frac{29}{15}\)

=>\(x\cdot\frac{3}{4\cdot5\cdot7}=\frac{29}{15}\)

=>\(x=\frac{29}{15}:\frac{3}{140}=\frac{29}{15}\cdot\frac{140}{3}=\frac{29\cdot28}{3\cdot3}=\frac{812}{9}\)

\(x:\dfrac{13}{16}=\dfrac{5}{-8}\\ x=\dfrac{5}{-8}.\dfrac{13}{16}=\dfrac{65}{-128}\)

\(x.\dfrac{-1}{2}=\dfrac{-4}{5}\\ x=\dfrac{-4}{5}:\dfrac{-1}{2}=\dfrac{8}{5}\)

Lớp 8A trồng được số cây là:
300.15%=45(cây)

Lớp 8B trồng được số cây là:

(300-45).\(\frac{1}{5}\)=51 (cây)

Gọi số cây của được giao của 3 lớp 8C,8D,8E là a,b,c(a,b,c>0)

Theo đề bài ta có: \(\frac{a}{8}=\frac{b}{4}=\frac{c}{5}\), a+b+c=300-45-51=204

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

\(\frac{a}{8}=\frac{b}{4}=\frac{c}{5}=\frac{a+b+c}{8+4+5}=\frac{204}{17}=12\)

\(\Rightarrow\hept{\begin{cases}a=12.8=96\\b=12.4=48\\c=12.5=60\end{cases}}\)

Vậy.....

a: \(x+6\dfrac{1}{8}=8\)

=>\(x+\dfrac{49}{8}=\dfrac{64}{8}\)

=>\(x=\dfrac{64}{8}-\dfrac{49}{8}=\dfrac{15}{8}\)

b: \(\dfrac{11}{2}\cdot x=\dfrac{1}{5}:\dfrac{1}{3}\)

=>\(x\cdot\dfrac{11}{2}=\dfrac{1}{5}\cdot3=\dfrac{3}{5}\)

=>\(x=\dfrac{3}{5}:\dfrac{11}{2}=\dfrac{3}{5}\cdot\dfrac{2}{11}=\dfrac{6}{55}\)

c: \(x\cdot\dfrac{3}{5}+\dfrac{2}{5}\cdot x=\dfrac{4}{9}+\dfrac{1}{3}\)

=>\(x\left(\dfrac{3}{5}+\dfrac{2}{5}\right)=\dfrac{4}{9}+\dfrac{3}{9}\)

=>\(x\cdot1=\dfrac{7}{9}\)

=>\(x=\dfrac{7}{9}\)

Giấy hơi nhàu, thông cảm :<