bài 75
a)5x^2.(3x^2-7x+2)
=15x^4-35x^3+10x^2
b)2/3xy(2x^2y-3xy+y^2)
=4/3x^3y^2-2x^2y^2+2/3xy63
BÀI 76
a)(2x^2-3x)(5x^2-2x+1)
10x^4-4x^3+2x^2-15x^3+6x^2-3x
10x^4-19x^3+8x^2-3x
b)(x-2y)(3xy+5y^2+x)
3x^2y+5xy^2+x^2-6xy^2-10y^3-2xy
3x^2y-xy^2+x^2-10y^3-2xy
bài 77
a)M=x^2+4y^2-4xy=x^2-4xy+4y^2=x^2-2x(2y)+(2y)^2=(x-2y)^2 tại x=18; y=4 thì m=(18-2.4)^2=10^2=100
b)N=8x^3-12x^2y+6xy^2-y^3=(2x)^3-3(2x)^2y+3.2.x.Y62-y^3=(2x-y)^3 tại x=6;y=-8 thì (2.6+8)^3=20^3=8000
bài 78
a)(x+2)(x-20-(x-30(x=1)
=(x^2-4)-(x^2+x-3x-3)
=x^2-4-x^2-x+3x+3
=2x-1
b)(2x+1)^2+(3x-1)^2+2(2x-1)-(3x-1)
=(2x+1)^2+2(2x-1)(3x-1)+(3x-1)^2
=[(2x+1)=(3x-1)]^2
=(2x+1+3x-1)^2=(5x)^2=25x^2
s.
