Xi由2) 令X=Xnailainra12a22anna21X'AX = (Xi,X2...Xn)...xnanannan2"WW"Waij,akj=(Xi,X2....Xn)amjt,85.1二次型的矩阵表示A§5.1 二次型的矩阵表示 1 2 , n x x X x     =       2) 令 由 11 12 1 1 21 22 2 2 1 2 1 2 ... ... ( , ,..., ) ... n n n n n nn n a a a x a a a x X AX x x x a a a x        =          1 1 2 1 2 1 1 ( , ,..., ) n j j j n j j n j n nj j j a x a x x x x a x = = =         =               