Tích chéo ta có:
\(-(2\frac{1}{2}x-1) ^2=-(\frac{2}{3})^2 \)
<=>\(2\frac{1}{2}x -1=\frac{2}{3} \)
<=>\(2\frac{1}{2}x =\frac{5}{3} \)
<=>\(\frac{5}{2}x=\frac{5}{3} \)
<=>\(x=\frac{5}{3}:\frac{5}{2} \)
<=>\(x=\frac{2}{3} \)
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