问答题
证明：秩等于r的对称矩阵可以表成r个秩等于1的对称矩阵之和.
问答题 x21+x22+x23+x24+2x1x2+2x2x3+2x3x4.
问答题 x21+2x22+x24+4x1x2+4x1x3+2x1x4+2x2x3+2x2x4+2x3x4.
问答题 x1x2+x1x3+x1x4+x2x3+x2x4+x3x4.
