\(\dfrac{x}{7}=\dfrac{6}{x+1}\) ĐK: x khác -1
<=> \(x\left(x+1\right)=6\cdot7\)
\(\Leftrightarrow x^2+x-42=0\)
\(\Leftrightarrow x^2-6x+7x-42=0\)
\(\Leftrightarrow x\left(x-6\right)+7\left(x-6\right)=0\)
\(\Leftrightarrow\left(x-6\right)\left(x+7\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-6=0\\x+7=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=6\\x=-7\end{matrix}\right.\)(tm)
Vậy.....
ĐKXĐ : x \(\ne-1\)
\(\Leftrightarrow\)x (x +1) = 42
\(\Leftrightarrow\)x2 + x - 42 = 0
\(\Leftrightarrow\)x2 - 6x + 7x - 42 = 0
\(\Leftrightarrow\)x ( x-6) + 7(x - 6)
\(\Leftrightarrow\)(x+7)(x-6)
