y=ax²+bx+c=a[x²+(b/a)x]+c=a[x²+(b/a)x+(b/2a)²]+c-a×(b/2a)²=a[x+(b/2a)]²+c-(b²/4a)=a[x+(b/2a)]²+(4ac-b²)/(4a)其中 h=-b/(2a),k=(4ac-b²)/(4a)