x²+5y²-4xy+2y+1=0
(x²-4xy+4y²)+(y²+2y+1)=0
(x-2y)²+(y+1)²=0
所以x=2y y=-1
x=-2
x²-y²=4-1=3加一个若a+b=3,b+c=4,a+c=5.则a²+b²+c²ab+bc+ac=a²+b²+c²+ab+bc+ac =(2a²+2b²+2c²+2ab+2bc+2ac)/2 =(a²+2ab+b²+a²+2ac+c²+b²+2bc+c²)/2 =[(a+b)²+(a+c)²+(b+c)²]/2 =(9+16+25)/2 =25