令a=,则2anxx=xx+anxx,于是 f(x1,x2,…,xn)=|1x2+a2x2+…+an un- +2a2x1x2+2a13x1x3+…+2a2 2 2 1 2 11 1 22 2 12 1 2 13 1 3 1, 1 2 11 1 12 1 2 1 1 2 21 2 1 22 2 2 2 2 1 1 2 2 , 1 2 2 2 ( , , , ) n nn n n n n n n n n n n n n n nn n n ij i j i j f x x x a x a 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 a x x a x x a x x a x a x x − − = = + + + + + + + = + + + + + + + + + + + + =  令 aij = aji，则 2 aij xi xj = aij xi xj + aji xi xj ，于是