Câu 13:
a) \(\dfrac{3}{5}-\dfrac{3}{4}\)
\(=\dfrac{3\cdot4}{5\cdot4}-\dfrac{3\cdot5}{4\cdot5}\)
\(=\dfrac{12}{20}-\dfrac{15}{20}\)
\(=-\dfrac{3}{20}\)
b) \(\dfrac{1}{7}\cdot\dfrac{-3}{8}+\dfrac{1}{7}\cdot\dfrac{-13}{8}\)
\(=\dfrac{1}{7}\cdot\left(-\dfrac{3}{8}-\dfrac{13}{8}\right)\)
\(=\dfrac{1}{7}\cdot-\dfrac{16}{8}\)
\(=\dfrac{1}{7}\cdot-2\)
\(=-\dfrac{2}{7}\)
Câu 14:
a) \(\dfrac{1}{4}+\dfrac{3}{4}x=-\dfrac{13}{8}\)
\(\Rightarrow\dfrac{3}{4}x=-\dfrac{13}{8}-\dfrac{1}{4}\)
\(\Rightarrow\dfrac{3}{4}x=-\dfrac{15}{8}\)
\(\Rightarrow x=-\dfrac{15}{8}:\dfrac{3}{4}\)
\(\Rightarrow x=-\dfrac{5}{2}\)
b) \(\left|x-\dfrac{5}{2}\right|-\dfrac{1}{2}=\dfrac{9}{2}\)
\(\Rightarrow\left|x-\dfrac{5}{2}\right|=\dfrac{9}{2}+\dfrac{1}{2}\)
\(\Rightarrow\left|x-\dfrac{5}{2}\right|=5\)
\(\Rightarrow\left[{}\begin{matrix}x-\dfrac{5}{2}=-5\left(x< \dfrac{5}{2}\right)\\x-\dfrac{5}{2}=5\left(x\ge\dfrac{5}{2}\right)\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-\dfrac{5}{2}\left(tm\right)\\x=\dfrac{15}{2}\left(tm\right)\end{matrix}\right.\)
15:
a: \(\dfrac{1}{4}=0,25;-\dfrac{5}{6}=-0,8\left(3\right);\dfrac{13}{50}=0,25;-\dfrac{17}{125}=-0,136\)
b:
2h40p chiều=14h40p
mẹ bạn Hoa phải di chuyển từ nhà ít nhất là từ:
14h40p-2h-45p=14h40p-2h45p=11h55p
Câu 14:
a: \(x-\dfrac{3}{5}=0,2\)
=>x-0,6=0,2
=>x=0,2+0,6=0,8
b: \(\left|x-2\right|=5\)
=>\(\left[{}\begin{matrix}x-2=5\\x-2=-5\end{matrix}\right.\)
=>\(\left[{}\begin{matrix}x=7\\x=-3\end{matrix}\right.\)
