1x1+a12x2+…+a1nn x1+a22x2+…+a2m 2 1n1+a2ny2+…+ ar 1a12 12 22 2n 2 2 n n nn 则二次型可记作f=xAx,其中A为对称矩阵 矩阵表示 K心( ) 1 1 2 2 1 2 1 2 2 2 2 1 1 1 1 2 2 1 1 2             + + + + + + + + + = 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        2 1 1 2 1 2 2 2 2 1 1 1 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          A x 则二次型可记作 f = xAx,其中A为对称矩阵. 矩阵表示