Giải:
a) \(3x^2y-6xy^2\)
\(=3xy\left(x-2y\right)\)
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b) \(\left(2x-a\right)x^2-\left(2x-a\right)y\)
\(=\left(2x-a\right)\left(x^2-y\right)\)
\(=\left(2x-a\right)\left(x-\sqrt{y}\right)\left(x+\sqrt{y}\right)\)
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c) \(25a^2-c^2\)
\(=\left(5a-c\right)\left(5a+c\right)\)
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d) \(4-36x+81x^2\)
\(=2^2-2.2.9x+\left(9x\right)^2\)
\(=\left(2-9x\right)^2\)
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e) \(\left(x+7\right)2-\left(2x-9\right)2\)
\(=2\left[\left(x+7\right)-\left(2x-9\right)\right]\)
\(=2\left(x+7-2x+9\right)\)
\(=2\left(16-x\right)\)
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f) \(x^2-6x+8\)
\(=x^2-6x+9-1\)
\(=\left(x-3\right)^2-1\)
\(=\left(x-4\right)\left(x-2\right)\)
Vậy ...