6){(xy)²+3y=45
{5y-2xy=3⇒2xy=(5y-3)⇒xy=(5y-3)/2;
x=(5y-3)/2;
(5y-3)²/2²+3y=45 ⇒25y²-30y+9+12y=180;⇒
25y²-18y-171=0 ⇒y₁₂=(18⁺₋√(18²+4·25·171))/50;
y₁=(18+132)/50=150/50=3; y₂=(18-132)/50=-114/50=-2.28;
x₁=(5·3-3)/2=6; x₂=(5·(-2.28)-3)/2=-14.4/2=-7.2.
7){(x+2y)²-2(x-2y)=11
{5(x+2y)+x-2y=-18 ⇒5x+10y+x-2y=-18⇒6x+8y=-18⇒6x=-(8y+18)⇒
x=-(8y+18)/6;
(-8y-18+12y)²/36-2·(-8y-18-12)/6=11⇒
(4y-18)²/36 +(8y+30)/3=11⇒(4y-18)²+12(8y+30)-11·36=0⇒
16y²-144y+324+96y+360-396=0⇒
16y²-48y+288=0⇒y²-3y+18=0
y₁₂=3/2⁺₋√9/4-18⇒D<0 корней нет