3.二次型及标准形与矩阵表示 (1)f(x1,x2,…,xn)=a1x12+2a12x1x2+…+2a1nx1xn a 22 +2a2x2x3+…+2a2nxxX 2 a n-1,n-1-n-1 +2 n-lmtn-1xn 2 +a, 11 n 12 22 2n 12 x Ar n K心3. 二次型及标准形与矩阵表示 ( ) 2 1, 1 2 1, 1 1 2 3 2 3 2 2 2 2 2 2 1 2 1 2 1 1 2 1 2 1 1 1 2 2 2 (1) , , , 2 2 nn n n n n n n n n n n n n n a x a x a x x a x a x x a x x f x x x a x a x x a x x + + + + + + + + = + + + − − − − −       x Ax x x x a a a a a a a a a x x x n n nn n n n n =                          = 2 1 1 2 1 2 2 2 2 1 1 1 2 1 1 2         