解方程组①x2-y2=3x+3y②x2-xy+y2=27

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解方程组①x2-y2=3x+3y②x2-xy+y2=27
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解方程组①x2-y2=3x+3y②x2-xy+y2=27
解方程组①x2-y2=3x+3y②x2-xy+y2=27

解方程组①x2-y2=3x+3y②x2-xy+y2=27
x²-y²=3x+3y①
x²-xy+y²=27②
①式得:(x+y)(x-y)=3(x+y)移项得:(x+y)(x-y)-3(x+y)=(x+y)(x-y-3)=0
则有x+y=0或x-y-3=0,即x=-y或x=y+3……③
将x=-y代入②有:(-y)²-(-y)y+y²=y²+y²+y²=3y²=27,同除以3,y²=9,则y=士3.
当y=3时,x=-y=-3;当y=-3时,x=3
即x1=-3、y1=3;x2=3,y2=-3.
x=y+3代入②有:.(y+3)²-(y+3)y+y²=y²+6y+9-y²-3y+y²=27
化简:y²+3y-18=(y+6)(y-3)=0
若y+6=0,y=-6,则x=y+3=-6+3=-3.
若y-3=0,y=3,x=y+3=3+3=6.
即x3=-3、y3=-6;x4=6、y4=3.
二元二次方程组共四组解.
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解:
①x2-y2=3x+3y
x-y=3
②x2-xy+y2=27
(x-y)^2+xy=27
xy=18
所以解得x=6
y=3
或x=-3
y=-6

x²-y²=3x+3y①
x²-xy+y²=27②