3) x³y +xy³ =xy(x²+y²) =xy *( (x-y)² +2xy ) =7(5²+2*7) =7*(25+14) =7*39 =273.
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4) x²+2xy +y² = x² -2xy + y² +4xy= 9+4*10 =49 =7²;
(x+y)² =7²⇔|x+y| =7.
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5) { x+y=6 ; x² -y² =12 .⇔ { x+y=6 ;(x +y) (x -y) =12 .⇔ { x+y=6 ;6(x -y) =12 .⇔
{ x+y=6 ;x -y =2 . ⇒(x+y) +(x-y)=6 +2⇔2x =8 ⇒ x=4.