(上定关孝 2.用矩阵表示 f =x1(1x1+a12X,+…+a1x,)+ 11 1 x2(a21x1+a2x2+…+a2nn)+…+ x(anx, +an2x2+.+anxn) 1x1+a12x2+……+a1nx a21x1+a22X2+…+a2 11 x19X2,…9Xn n1x1+an2X2+…+amXn n n x ax 1,~2 n nI n2 n 记为A称为二次型的矩阵,2．用矩阵表示 1 11 1 12 2 1 2 21 1 22 2 2 1 1 2 2 ( ) ( ) ( ) n n n n n n n nn n f x a x a x a x x a x a x a x x a x a x a x = + + + + + + + + + + + +       + + + + = n n nn n n n n n n a x a x a x a x a x a x a x a x a x x x x      1 1 2 2 21 1 22 2 2 11 1 12 2 1 1 2 ( , , , ) ( )             = n n nn n n n n x x x a a a a a a a a a x x x          2 1 1 2 21 22 2 11 12 1 1 2 , , , T = x Ax 记为A,称为二次型的矩阵， A为对称矩阵 记为x