简单计算一下即可，答案如图所示
f(X1,X2,X3)=X1^2+2X3^2+2X1X3=(X1+X3)^2+X3^2令Y1=X1+X3Y2=X2Y3=Y3则f(X1,X2,X3)=Y1^2+Y3^2=g(Y1,Y2,Y3)X=PY其中变换矩阵P是1 0 -10 1 00 0 1
