f = (x1+x2-2x3)^2+2x2^2+x3^2+4x2x3
= (x1+x2-2x3)^2+2(x2+x3)^2 - x3^2
= y1^2+2y2^2-y3^2.
Y=CX,其中变换矩阵 C =
1 0 0
1 1 0
-2 1 1