Bài 3:
1) \(\left(3x^2-4\right)\left(x+3y\right)\)
\(=3x^3+9x^2y-4x-12y\)
2) \(\left(x+3\right)\left(x^2+3x\right)\)
\(=x^3+3x^2+3x^2+9x\)
\(=x^3+6x^2+9x\)
3) \(\left(xy-1\right)\left(xy+5\right)\)
\(=x^2y^2+5xy-xy-5\)
\(=x^2y^2+4xy-5\)
4) \(\left(3x+5y\right)\left(2x-7y\right)\)
\(=6x^2-21xy+10xy-35y^2\)
\(=6x^2-11xy-35y^2\)
5) \(-\left(x-1\right)\left(-x^2+2y\right)\)
\(=\left(x-1\right)\left(x^2-2y\right)\)
\(=x^3-2xy-x^2+2y\)
6) \(\left(-x^2+2y\right)\left(x^2+2y\right)\)
\(=-\left(x^2-2y\right)\left(x^2+2y\right)\)
\(=-\left(x^4-4y^2\right)\)
\(=-x^4+4y^2\)
7) \(\left(x+3y\right)\left(x-3y+2\right)\)
\(=x^2-3xy+2x+3xy-9y^2+6y\)
\(=x^2+2x-9y^2+6y\)
8) \(\left(x+2y\right)\left(x-2y+3\right)\)
\(=x^2-2xy+3x+2xy-4y^2+6y\)
\(=x^2+3x-5y^2+6y\)
9) \(\left(x^2-xy+y^2\right)\left(x+y\right)\)
\(=x^3+y^3\)
10) \(\left(x^2-xy+y^2\right)\left(x+y\right)\)
\(=x^3+y^3\)
