\(4x^2-81=0\)
\(\Rightarrow\left(2x\right)^2-9^2=0\)
\(\Rightarrow\left(2x-9\right).\left(2x+9\right)=0\)
\(\Rightarrow\orbr{\begin{cases}2x-9=0\\2x+9=0\end{cases}\Rightarrow\orbr{\begin{cases}x=\frac{9}{2}\\x=-\frac{9}{2}\end{cases}}}\)
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\(4x^2-81=0\)
\(\Leftrightarrow\left(2x\right)^2-9^2=0\)
\(\Leftrightarrow\left(2x-9\right)\left(2x+9\right)=0\)
\(2x-9=0\)
\(2x=9\)
\(x=\frac{9}{2}\)
\(2x+9=0\)
\(2x=-9\)
\(x=-\frac{9}{2}\)
b)(x-2)(x+2)-x(x-8)=10
\(x^2-4-x^2+8x=10\)
\(8x-4=10\)
\(8x=14\)
\(x=\frac{7}{4}\)
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