\(4x^2-81=0\Leftrightarrow4x^2=81\Leftrightarrow x^2=\frac{81}{4}\Leftrightarrow x=\pm\frac{9}{2}\)
\(4x^2-81=0\)
\(4x^2=81\)
\(x^2=81:4\)
\(x^2=\left(\frac{9}{2}\right)^2\)
\(\Rightarrow x=\frac{9}{2}\)
Học tốt
4x2 - 81 = 0
<=> ( 2x )2 - 92 = 0
<=> ( 2x - 9 )( 2x + 9 ) = 0
<=> \(\orbr{\begin{cases}2x-9=0\\2x+9=0\end{cases}}\)
<=> \(\orbr{\begin{cases}2x=9\\2x=-9\end{cases}}\)
<=> \(\orbr{\begin{cases}x=\frac{9}{2}\\x=-\frac{9}{2}\end{cases}}\)
