a)$ C=A+B= x^2+2xy-4y^2 + 5y^2+2xy+2x^2-1$
$= 3x^3 + 4xy + y^2 -1 $
Bậc: $3$
.
b) $D=A-B = x^2+2xy-4y^2 - (5y^2+2xy+2x^2-1)$
$= x^2+2xy-4y^2 - 5y^2 - 2xy - 2x^2 + 1$
$= -x^2 - 9y^2 +1$
Bậc: $2$
.
c) Tại$ x=1; y=-1$
=> C= 3x^3 + 4xy + y^2 -1 = 3.1^3 + 4.1.(-1) + (-1)^2 -1
$= 3 - 4 +1 -1$
$= -1$
.
Tại $x=1; y=-1$
=> $D= -x^2 - 9y^2 +1= -1^2 - 9.(-1)^2 +1$
$= -1 - 9 +1$
$= -9$