Bài 2:
A.B=(2xyz²).($\frac{-1}{2}$ x²y²)
= -x³y³z²
Bài 4:
a) P(x)= -7+4x+5x³
= 5x³+4x-7
Q(x)= -2-4x²+2x-3+3x²+2x+5x³
= -(2+3)+(3x²-4x²)+(2x+2x)+5x³
= -5-x²+4x+5x³
= 5x³-x²+4x-5
b) P(x)+Q(x)= 5x³+4x-7+5x³-x²+4x-5
= (5x³+5x³)-x²+(4x+4x)-(7+5)
= 10x³-x²+8x-12
H(x)= 5x³+4x-7-(5x³-x²+4x-5)
= 5x³+4x-7-5x³+x²-4x+5
=(5x³-5x³)+x²+(4x-4x)+(5-7)
= x²-2
c) H($\frac{-1}{2}$ )= ($\frac{-1}{2}$ )²-2=$\frac{-7}{4}$
Bài 5:
a) 2x²y².$\frac{1}{4}$ xy³.(-3xy)
=$\frac{-3}{2}$ $x^{4}$ $y^{6}$
b) (-2x³y)².xy².$\frac{1}{2}$ y³
=4$x^{6}$ y².xy².$\frac{1}{2}$ y³
= 2$x^{7}$ $y^{7}$
