f(x1,x2…,xn)=a1x1+12xx2+…+anxx, +a21x2x1+a22x2+…+a2nx2X +an1xx1+a.xx2+…+ax =(x1.x x 2 n 11 记 x=2则二次型可记作 xn f=x Ax( ) 11 12 1 1 21 11 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          =          2 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 = + + + + + + + + + + + + 11 12 1 21 11 2 1 2 n n n n nn a a a a a a A a a a       =       1 2 n x x x       =       记 x 则二次型可记作 T f A = x x