a.
\(A=\left(3x^3-6x^2y^2\right)+\left(xy-2y^3\right)\)
\(=3x^2\left(x-y^2\right)+y\left(x-2y^2\right)\)
\(=\left(3x^2+y\right)\left(x-2y^2\right)\)
\(=\left(3x^2+y\right).0=0\)
b.
\(B=\left(4x^2-12xy+9y^2\right)-\left(4x^2-25\right)+12xy-9y^2\)
\(=4x^2-12xy+9y^2-4x^2+25+12xy-9y^2\)
\(=25\) với mọi x;y
c.
\(C=x^4+x^3+x^2y^2+xy^3-x^3y-x^2y-xy^3-y^4-x^4+y^4\)
\(=x^3+x^2y^2-x^3y-x^2y\)
\(=x^3\left(1-y\right)-x^2y\left(1-y\right)\)
\(=\left(x^3-x^2y\right)\left(1-y\right)\)
Khi \(y=1\Rightarrow1-y=0\)
\(\Rightarrow C=\left(x^3-x^2y\right).0=0\)
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