问答题8x1x4+2x3x4+2x2x3-8x2x4.
问答题x21+3x22-2x1x2+2x1x3-6x2x3.
问答题x21+2x1x2+2x22+4x2x3+4x23.
问答题-4x1x2+2x1x3+2x2x3
问答题证明:设m×n矩阵A的秩为r,则有m×r的列满秩矩阵P和r×n的行满秩矩阵Q,使A=PQ.