, 12X )=x1(a1x1+a12x2+…+a1nxn) +x2(a21x1+a2x2+…+a2nxn) fx x1+a.x++.x n n1 +ax 122 …a, 21X1+2x+十,nX , 19299n 对称阵 n1x1+an2x2+…+amCn 12 22 n , 1~255叫n =x Ax2 1 2 11 1 12 1 2 1 1 2 21 2 1 22 2 2 2 2 1 1 2 2 ( , , , ) n n n n n n n n n nn n f x x x a x a x x a x x a x x a x a x x a x x a x 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 1 1 2 2 ( ) n n n nn n + + + + x a x a x a x 11 1 12 2 1 21 1 22 2 2 1 2 1 1 2 2 ( , , , ) n n n n n n n nn n a x a x a x a x a x a x x x x a x a x a x   + + +   + + +   =     + + +   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 x x a a a x          =       T    = x Ax 对称阵