Tìm , biết rằng : a) b) c)

Bài 16 (Sách bài tập - tập 1 - trang 9)

Gửi bởi: Sách Giáo Khoa Vào 26 tháng 12 2018 lúc 11:16:40

Tìm \(x\in Q\), biết rằng :

a) \(\dfrac{11}{12}-\left(\dfrac{2}{5}+x\right)=\dfrac{2}{3}\)

b) \(2x\left(x-\dfrac{1}{7}\right)=0\)

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

a)\(\dfrac{11}{12}-\left(\dfrac{2}{5}+x\right)=\dfrac{2}{3}\)

\(\dfrac{2}{5}+x=\dfrac{11}{12}-\dfrac{2}{3}\)

\(\dfrac{2}{5}+x=\dfrac{11}{12}-\dfrac{8}{12}\)

\(\dfrac{2}{5}+x=\dfrac{3}{12}\)

\(\dfrac{2}{5}+x=\dfrac{1}{4}\)

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

\(x=\dfrac{5}{20}-\dfrac{8}{20}\)

\(x=\dfrac{-3}{20}\)

b)\(2x\left(x-\dfrac{1}{7}\right)=0\)

\(\Rightarrow2x=0\) hoặc \(x-\dfrac{1}{7}=0\)

\(x=0:2\) \(x=0+\dfrac{1}{7}\)

\(x=0\) \(x=\dfrac{1}{7}\)

\(\Rightarrow x=0\) hoặc \(x=\dfrac{1}{7}\)

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

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

\(\dfrac{1}{4}:x=\dfrac{8}{20}-\dfrac{15}{20}\)

\(\dfrac{1}{4}:x=\dfrac{-7}{20}\)

\(x=\dfrac{1}{4}:\dfrac{-7}{20}\)

\(x=\dfrac{1}{4}.\dfrac{-20}{7}\)

x= \(\dfrac{1.\left(-5\right)}{1.7}\)

\(x=\dfrac{-5}{7}\)