Question

Bài 1: Tính nhanh

      a, \(\frac{-3}{7}-\left(\frac{2}{3}-\frac{3}{7}\right)\)         

      b, \(\frac{2}{15}:\left(\frac{1}{3}.\frac{4}{5}-\frac{1}{3}.\frac{6}{5}\right)\)

Bài 2: Tìm x:

        a, \(\frac{3}{4}+\frac{1}{4}.x=\frac{5}{8}\)

        b, 25% . \(x\)+\(x\)= -1,25

Answer 1

a) \(\frac{-3}{7}-\left(\frac{2}{3}-\frac{3}{7}\right)\)

\(=\frac{-3}{7}-\frac{2}{3}+\frac{3}{7}\)

\(=\left(\frac{3}{7}-\frac{3}{7}\right)-\frac{2}{3}\)

\(=0-\frac{2}{3}\)

\(=-\frac{2}{3}\)

b) \(\frac{2}{15}\div\left(\frac{1}{3}.\frac{4}{5}-\frac{1}{3}.\frac{6}{5}\right)\)

\(=\frac{2}{15}\div\left[\frac{1}{3}\left(\frac{4}{5}-\frac{6}{5}\right)\right]\)

\(=\frac{2}{15}\div\left[\frac{1}{3}.\frac{-2}{5}\right]\)

\(=\frac{2}{15}\div\frac{-2}{15}\)

\(=\frac{2}{15}\times\frac{15}{-2}\)

\(=\frac{2}{-2}=-1\)

Answer 2

\(\frac{3}{4}+\frac{1}{4}x=\frac{5}{8}\)

\(\Leftrightarrow\frac{1}{4}x=\frac{5}{8}-\frac{3}{4}\)

\(\Leftrightarrow\frac{1}{4}x=\frac{5}{8}-\frac{6}{8}\)

\(\Leftrightarrow\frac{1}{4}x=\frac{-1}{8}\)

\(\Leftrightarrow x=\frac{-1}{8}\div\frac{1}{4}\)

\(\Leftrightarrow x=\frac{-1}{8}\times\frac{4}{1}\)

​\(\Leftrightarrow x=\frac{-4}{8}\)​

\(\Leftrightarrow x=\frac{-1}{2}\)

Answer 3

\(25\%x+x=-1,25\)

\(\Leftrightarrow\frac{25}{100}x+x=\frac{-125}{100}\)

\(\Leftrightarrow\frac{1}{4}x+x=\frac{-25}{20}\)

\(\Leftrightarrow x\left(\frac{1}{4}+1\right)=\frac{-5}{4}\)

\(\Leftrightarrow x\left(\frac{1}{4}+\frac{4}{4}\right)=-\frac{5}{4}\)

\(\Leftrightarrow\frac{5}{4}x=\frac{-5}{4}\)

\(\Leftrightarrow x=\frac{-5}{4}\div\frac{5}{4}\)

\(\Leftrightarrow x=\frac{-5}{4}\times\frac{4}{5}\)

\(\Leftrightarrow x=\frac{-5}{5}=-1\)