|a1+2a2,a3,a1+a2|=|a1+a2+a2,a3,a1+a2|
=|a1+a2,a3,a1+a2|+|a2,a3,a1+a2|=0+|a2,a3,a1+a2|(因为两列相等行列式为0)
=|a2,a3,a1+a2|=|a2,a3,a1|+|a2,a3,a2|=|a2,a3,a1|+0(因为两列相等行列式为0)
=|a2,a3,a1|
=-|a2,a1,a3|(交换行列式的两列行列式异号)
=|a1,a2,a3|(交换行列式的两列,行列式异号)
|a1+2a2,a3,a1+a2|=|a1+a2+a2,a3,a1+a2|
=|a1+a2,a3,a1+a2|+|a2,a3,a1+a2|=0+|a2,a3,a1+a2|(因为两列相等行列式为0)
=|a2,a3,a1+a2|=|a2,a3,a1|+|a2,a3,a2|=|a2,a3,a1|+0(因为两列相等行列式为0)
=|a2,a3,a1|
=-|a2,a1,a3|(交换行列式的两列行列式异号)
=|a1,a2,a3|(交换行列式的两列,行列式异号)