设二次型f(x1,x2,x3)=2(a1x1+a2x2+a3x3)2 +(b1x1+b2x2+b3x3)2 , 记 证明二次型,对应的矩阵为2ααT +ββT ;

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问题 设二次型f(x1,x2,x3)=2(a1x1+a2x2+a3x3)2 +(b1x1+b2x2+b3x3)2 ,


证明二次型,对应的矩阵为2ααT +ββT

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答案f=(2a12+b12)x12+(2a22+b22)x22+(2a32+b32)x32+(4a12a22+2b12b22)x1x2+(4a1a3+2bb)x1x3+(4a2a3+2b2b3)x2x3 则f的矩阵为 [*] =XT(2ααT)X+XT(2ββT)X=XT(2ααT+ββT)X 所以f的矩阵=2ααT +ββT

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