f(x1,x2,…,xn)=a1x1+a12x1x2+…+a12x1xn x1+a2x2+…+a2nx2xn+ axx+ tax ∑∑ax D•• , , , , = = =+ + + + + + + = + + + + ni nj i j i j n n n n n n n n n n n n a x x a x x a x x a x a x x a x a x x f x x x a x a x x a x x 1 1 2 1 1 2 2 2 2 2 2 1 2 1 2 2 2 1 2 1 2 1 1 2 1 2 1 1 1 ( )