用配方法化二次型f(x1，x2，x3)=2x12+2x32+2x32+2x1x2+2x1x3+2x2x3为标准形．
f=2x12+2x22+2x32+2x1x2+2x1x3+2x2x3 =x12+2x1x2+x22+x12+2x1x3+x2+x22+2x2x3+x32 =(x1+x2)2+(x1+x3)2+(x2+x3)2， 令 {y1=x1+x2 {y2=x1+x3 {y3=x2+x3 即 {x1=(1/2)y1+(1/2)y2-(1/2)y3 {x2=(1/2)y1-(1/2)y2+(1/2)y3 {x3=-(1/2)y1+(1/2)y2+(1/2)y3 则f=y12+y22+y32
