a) Ta có : \(4x^2-10x+9=0\)
\(\Rightarrow\left(2x\right)^2-2.2x.\frac{5}{2}+\left(\frac{5}{2}\right)^2+\frac{11}{2}=0\)
\(\Rightarrow\left(2x-\frac{5}{2}\right)^2+\frac{11}{2}=0\)(vô lý)
\(\Rightarrow4x^2-10+9\)vô nghiệm(đpcm)
b) Ta có: \(-1+x-x^2=0\)
\(\Rightarrow\left(-1+x-x^2\right).\left(-1\right)=0\)
\(\Rightarrow x^2-x+1=0\)
\(\Rightarrow x^2-2.x.\frac{1}{2}+\left(\frac{1}{2}\right)^2+\frac{3}{4}=0\)
\(\Rightarrow\left(x-\frac{1}{2}\right)^2+\frac{3}{4}=0\)(vô lý)
\(\Rightarrow-1+x-x^2\) vô nghiệm(đpcm)
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