Bài 1:
\(a)7x^2-21x=0.\\ \Leftrightarrow7x\left(x-3\right)=0.\\ \Leftrightarrow\left[{}\begin{matrix}7x=0.\\x-3=0.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0.\\x=3.\end{matrix}\right.\)
\(b)x^2-36=0.\\ \Leftrightarrow\left(x-6\right)\left(x+6\right)=0.\\ \Leftrightarrow\left[{}\begin{matrix}x-6=0.\\x+6=0.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=6.\\x=-6.\end{matrix}\right.\)
\(c)x^2+6x-16=0.\\ \left(x-2\right)\left(x+8\right)=0.\\ \Leftrightarrow\left[{}\begin{matrix}x-2=0.\\x+8=0.\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=2.\\x=-8.\end{matrix}\right.\)
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