配方后平方项可顺序命名为 y1,y2,y3,...
逐一回代即可
x3=y3
x2 = y2-x3 = y2-y3
x1 = y1-2x2-3x3 = y1 - 2(y2-y3) -3y3 = y1 -2y2 -y3