4: 1/x là số nguyên khi 1⋮x
=>x∈{1;-1}
3:
a: \(\frac23x+\frac57=\frac{3}{10}\)
=>\(\frac23x=\frac{3}{10}-\frac57=\frac{21-50}{70}=\frac{-29}{70}\)
=>\(x=-\frac{29}{70}:\frac23=-\frac{29}{70}\cdot\frac32=\frac{-87}{140}\)
b: \(\frac54x-\frac12=\frac37\)
=>\(\frac54x=\frac12+\frac37=\frac{7+6}{14}=\frac{13}{14}\)
=>\(x=\frac{13}{14}:\frac54=\frac{13}{14}\cdot\frac45=\frac{52}{70}=\frac{26}{35}\)
d: \(x\cdot x=x\)
=>x(x-1)=0
=>\(\left[\begin{array}{l}x=0\\ x-1=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=0\\ x=1\end{array}\right.\)
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