`P - Q = (-x^2 y + 3xy^2 - 1) - (-2x^2 y + 4xy^2 - 5)`
`= -x^2 y + 3xy^2 - 1 +2x^2 y - 4xy^2 + 5`
`= x^2 y - xy^2 + 4 ` (chọn C)
(ab^2 - a^2 b)(a+b+ab)
`= a^2 b^2 + ab^3 + a^2 b^3 - a^3 b - a^2 b^2 - a^3 b^2`
`= + ab^3 + a^2 b^3 - a^3 b - a^3 b^2`
(chọn D)
Câu 4: \(P-Q=-x^2y+3xy^2-1+2x^2y-4xy^2+5\)
\(=x^2y-xy^2+4\)
=>Chọn C
Câu 5: \(\left(ab^2-a^2b\right)\left(a+b+ab\right)\)
\(=ab\left(b-a\right)\left(b+a+ab\right)\)
\(=ab\left(b^2+ba+ab^2-ab-a^2-a^2b\right)\)
\(=ab\left(b^2-a^2+ab^2-a^2b\right)\)
\(=ab^3-a^3b+a^2b^3-a^3b^2\)
=>Chọn D
\(P-Q=-x^2y+3xy^2-1-\left(-2x^2y+4xy^2-5\right)=-x^2y++3xy^2-1+2x^2y-4xy^2+5=x^2y-xy^2+4\)chọn C
\(\left(ab^2-a^2b\right).\left(a+b+ab\right)=ab^2\left(a+b+ab\right)-a^2b.\left(a+b+ab\right)=a^2b^2+ab^3+a^2b^3-a^3b-a^2b^2-a^3b^2=ab^3+a^2b^3-a^3b-a^3b^2\)chọn D
