tìm x;y

S

songdzianhem

H

huynhbachkhoa23

Bài 1:

$\leftrightarrow (x-y)(x^2+y^2+xy)+2xy(x-y)=1991$

$\leftrightarrow (x-y)(x^2+y^2+3xy)=1991$

$x=y$ hoặc $x^2+3yx+y^2=1991$

Bài 2:

$\leftrightarrow (2x+2y)^2+(x-1)^2+(y+1)^2=0$

Suy ra $x=-y=1$

Bài 3:

$A=(x^2+cx+d)^2 = x^4+c^2x^2+d^2+2cx^3+2dx^2+2cdx$

$=x^4+2cx^3+(c^2+2d)x^2+2cdx+d^2$

Suy ra:
$c=-1$
$1+2d=3$
$2cd=a$
$b=d^2$

Hay $a=-2; b=1$
 
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