Question

Tìm x:

\(3\left(3x-5\right)=9x^2-25\)

Answer 1

Ta có:

\(3\left(3x-5\right)=9x^2-25\\ \Leftrightarrow9x^2-9x-10=0\\ \Leftrightarrow\left(3x\right)^2-2.3x.\dfrac{3}{2}+\dfrac{9}{4}=\dfrac{49}{4}\\ \Leftrightarrow\left(3x-\dfrac{3}{2}\right)^2=\dfrac{49}{4}\\ \Leftrightarrow\left[{}\begin{matrix}3x-\dfrac{3}{2}=\dfrac{7}{2}\\3x-\dfrac{3}{2}=\dfrac{-7}{2}\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}3x=5\\3x=-2\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{5}{3}\\x=\dfrac{-2}{3}\end{matrix}\right.\)

Answer 2

3(3x-5)= (3x-5)(3x+5)

3(3x-5)-(3x-5)(3x+5)=0

(3-3x+5)(3x+5)=0

(-3x+8)(3x+5)=0

TH1 X=8/3

TH2 X=-5/3