1 ) \(\left(x^2-2x+5\right)\left(x-2\right)=\left(x^2+x\right)\left(x-5\right)\)
\(\Leftrightarrow x^3-2x^2+5x-2x^2+4x-10=x^3+x^2-5x^2-5x\)
\(\Leftrightarrow x^3-4x^2+9x-10=x^3-4x^2-5x\)
\(\Leftrightarrow9x-10=-5x\)
\(\Leftrightarrow14x=10\)
\(\Leftrightarrow x=\dfrac{5}{7}\)
Vậy \(x=\dfrac{5}{7}\)
2 ) \(\left(x-9\right)\left(x-9\right)+\left(2x+1\right)\left(2x+1\right)-\left(5x-4\right)\left(x-2\right)\)
\(=\left(x-9\right)^2+\left(2x+1\right)^2-\left[5x^2-4x-10x+8\right]\)
\(=x^2-18x+81+4x^2+4x+1-5x^2+4x+10x-8\)
\(=\left(x^2+4x^2-5x^2\right)+\left(4x+4x+10x-18x\right)+\left(81+1-8\right)\)\(=74\)
\(\Rightarrowđpcm\)
3 ) \(\left(2m-3\right)\left(3n-2\right)-\left(3m-2\right)\left(2n-3\right)\)
\(=6mn-9n-4m+6-\left[6mn-4n-9m+6\right]\)
\(=6mn-9n-4m+6-6mn+4n+9m-6\)
\(=9m-9n+4n-4m\)
\(=5m-5n\)
\(=5\left(m-n\right)⋮5\left(đpcm\right)\)
