=xax,+x,2a2x,+.+a..--(xag)=2ajxixji-l i=1f(X1,X2....x,)= X'AX于是有85.1二次型的矩阵表示§5.1 二次型的矩阵表示 于是有 1 2 ( , ,..., ) . n f x x x X AX =  1 1 2 2 1 1 1 n n n j j j j n nj j j j j x a x x a x x a x = = = = + + +    1 1 ( ) n n i ij j i j x a x = = =   1 1 n n i j ij i j a x x = = =  