Bài 7:
a: \(A=3x^2\left(6x^2+1\right)-9x\left(2x^3-x\right)\)
\(=18x^4+3x^2-18x^4+9x^2\)
\(=12x^2\)
b: \(B=x^2\left(x-2y\right)+2xy\left(x-y\right)+\frac13y^2\left(6x-3y\right)\)
\(=x^3-2x^2y+2x^2y-2xy^2+2xy^2-y^3\)
\(=x^3-y^3\)
Bài 6:
a: \(M=2x^2\left(1-3x+2x^2\right)\)
\(=2x^2\cdot1-2x^2\cdot3x+2x^2\cdot2x^2\)
\(=2x^2-6x^3+4x^4\)
b: \(N=\left(2x^2-3x+4\right)\left(-\frac12x\right)\)
\(=-\frac12x\cdot2x^2+\frac12x\cdot3x-\frac12x\cdot4\)
\(=-x^3+\frac32x^2-2x\)
c: \(P=\frac12xy\left(-x^2+2xy-4y^2\right)\)
\(=-\frac12xy\cdot x^2+\frac12xy\cdot2xy-\frac12xy\cdot4y^2\)
\(=-\frac12x^3y+x^2y^2-2xy^3\)
d: \(\frac12x^2y\left(2x^3-\frac25xy^2-1\right)\)
\(=\frac12x^2y\cdot2x^3-\frac12x^2y\cdot\frac25xy^2-\frac12x^2y\)
\(=x^5y-\frac15x^3y^3-\frac12x^2y\)
