mệt
refer
https://www.google.com/search?q=x2+-+25+-+8(5-x)+%3D+0&sourceid=chrome&ie=UTF-8
x2 - 25 - 40 + 8x = 0
<=> x2 + 8x - 65 = 0
<=> x(x + 8) -65 = 0
<=> (x - 65) ( x+8) = 0
\(\Leftrightarrow\left[{}\begin{matrix}x=65\\x=-8\end{matrix}\right.\)
\(x^2-25-8\left(5-x\right)=0\)
\(\Leftrightarrow x^2-25-8\left(-x+5\right)=0\)
\(\Leftrightarrow x^2-8\left(-x+5\right)-25=0\)
\(\Leftrightarrow x^2-\left(-8x+8\cdot5\right)-25=0\)
\(\Leftrightarrow x^2-\left(-8x+40\right)-25=0\)
\(\Leftrightarrow x^2+8x+40-25=0\)
\(\Leftrightarrow x^2+8x-65=0\)
\(\Leftrightarrow x^2-5x+13x-65=0\)
\(\Leftrightarrow x\left(\dfrac{x^2}{x}-\dfrac{5x}{x}\right)+13\left(\dfrac{13x}{13}-\dfrac{5\cdot13}{13}\right)=0\)
\(\Leftrightarrow x\left(x^{2-1}-5\right)+13\left(x-5\right)=0\)
\(\Leftrightarrow x-5\left(\dfrac{x\left(x-5\right)}{x-5}+\dfrac{13\left(x-5\right)}{x-5}\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x-5=0\\x+13=0\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x=5\\x=-13\end{matrix}\right.\)
x/x2-8 
1/3= 0.
