问答题
x1x2+x1x3+x1x4+x2x3+x2x4+x3x4.
问答题 8x1x4+2x3x4+2x2x3-8x2x4.
问答题 x21+3x22-2x1x2+2x1x3-6x2x3.
问答题 x21+2x1x2+2x22+4x2x3+4x23.