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\(9x^2-4\left(x-1\right)^2\)
\(=\left[3x+2\left(x-1\right)\right]\left[3x-2\left(x-1\right)\right]\)
\(=\left[3x+2x-2\right]\left[3x-2x+2\right]\)
\(=\left(5x-2\right)\left(x+2\right)\)
\(=\left(x^2+2x\right)^2+9\left(x^2+2x\right)+20\)
\(=\left(x^2+2x+4\right)\left(x^2+2x+5\right)\)
\(x^3-9x^2+14x\)
= \(x^3-7x^2-2x^2+14x\)
= \(x^2.\left(x-7\right)-2x.\left(x-7\right)\)
= \(\left(x-7\right).\left(x^2-2x\right)\)
= \(\left(x-7\right).\left(x-2\right).x\)
\(x^3-9x^2+14x\)
\(=x\left(x^2-9x+14\right)\)
\(=x\left(x^2-7x-2x+14\right)\)
\(=x\left[x\left(x-7\right)-2\left(x-7\right)\right]\)
\(=x\left(x-2\right)\left(x-7\right)\)
\(=x^3+2x^2-8x=x\left(x^2+2x-8\right)\\ =x\left(x^2-2x+4x-8\right)\\ =x\left(x-2\right)\left(x+4\right)\)
\(9x^2-12xy+4y^2\)
\(=\left(3x\right)^2-2.3x.2y+\left(2y\right)^2\)
\(=\left(3x-2y\right)^2\)
\(\left(x^2-3x+2\right)\left(x^2-9x+20\right)-40=\left(x-1\right)\left(x-2\right)\left(x-4\right)\left(x-5\right)-40\)
\(=\left(x^2-6x+5\right)\left(x^2-6x+8\right)-40\)
Đặt \(t=x^2-6x+5\) thì ta có \(t\left(t+3\right)-40=t^2+3t-40=\left(t+8\right)\left(t-5\right)\)
Suy ra \(\left(x^2-6x+5\right)\left(x^2-6x+8\right)-40=\left(x^2-6x+13\right)\left(x^2-6x\right)=x\left(x-6\right)\left(x^2-6x+13\right)\)
A=\(x^2-9x\)
\(=x\cdot x-x\cdot9\)
=x(x-9)