[简答题]判定以下二次型是不是正定二次型(需说明理由)：(1)f(x1，x2，x3=2x21+2x1x2-2x1x3-6x22一4x2x3-x23；(2)f(x1，x2，x3=2x21+2x1x2+4x1x3+2x22+2x2x3+3x23；(3)f(x1，x2，x3=2x21+2x1x2-4x1x3+2x222x2x3+5x23；(4)f(x1，x2，x3=x21+4x1x2+2x1x3+4x22+4x2x3+x23；(5)f(x1，x2，x3)=x21-2x1x24x1x3+2x224x2x3+7x23；(6)f(x1，x2，x3，x4)=x21+x22+4x23+8x24+6x1x3+4x1x4-2x2x3+2x2x4+2x3x4；(7)f(x1，x2，x3，x4)=x212x1x2+4x1x3+2x1x4+3x22-6x2x3+9x-12x2x4+19x24．