y=ax²+bx+c,化为顶点式是：y=a(x+b/2a)²+(4ac-b²)/4a
配方过程如下：y=ax²+bx+c
=a(x²+bx/a)+c
=a(x²+bx/a+b²/4a²-b²/4a²)+c
=a(x+b/2a)²-b²/4a+c
=a(x+b/2a)²+(4ac-b²)/4a
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