Bn cứ áp dụng hằng đẳng thức đi
a)\(x^2-5x=6\)
\(\Rightarrow x^2-2.\dfrac{5}{2}.x+\dfrac{25}{4}=6+\dfrac{25}{4}\)
\(\Rightarrow\left(x-\dfrac{5}{2}\right)^2=12,25\)
\(\Rightarrow x-\dfrac{5}{2}\in\left\{\pm3,5\right\}\)
\(\Rightarrow\left[{}\begin{matrix}x-\dfrac{5}{2}=3,5\\x-\dfrac{5}{2}=-3,5\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=6\\x=-1\end{matrix}\right.\)
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