f(x1,x2,…,xn)=a1x1+a12x1x2+…+a1nx1xn 2、二次型 ta21x2rta22x2+.ta2nt2x 的矩阵表示 taX n a 2 x(n1x1+12x2+…+a1nxn +x2(a21+a2x2+…+a2n 十 xn(anx1+mn2x2+…+mxn 11 21 22 n n2 n２、二次型 的矩阵表示 2 1 2 11 1 12 1 2 1 1 ( , , , ) n n n f x x x a x a x x a x x = + + + 2 21 2 1 22 2 2 2 n n + + + + a x x a x a x x + 2 n n n n nn n 1 1 2 2 + + + + a x x a x x a x = + + + x a x a x a x 1 11 1 12 2 1 ( n n ) + + + + x a x a x a x 2 21 1 22 2 2 ( n n ) + + + + + x a x a x a x n n n nn n ( 1 1 2 2 ) ( ) 11 11 11 1 21 22 2 2 1 2 1 2 n n n n nn n a a a x a a a x x x x a a a x       =             ③