例如：y=3x²+2x
先提取二次系数3可得：
y=3(x²+ 2x/3）
=3【x²+ 2*x*(1/3) + (1/3)² -(1/3)²]
=3【(x+ 1/3)²- (1/9)]
=3(x+ 1/3)²- (1/3)
一般的：
形如y=ax²+bx+c经过配方后可得：
y=a*[x+ b/(2a)]²+ (4ac-b²)/4a