C=(a1+2a2,2a2+3a4,a4+3a1)=(a1,a2,a4)K
K=
1 0 3
2 2 0
0 3 1
所以 40=|C|=|a1,a2,a4||K|=20|a1,a2,a4|
所以 |a1,a2,a4|=2.
|A|=|a1,a2,2a3-a4+a2|
c3-c2
= |a1,a2,2a3-a4|
= |a1,a2,2a3| + |a1,a2,-a4|
= -2|a3,a2,a1| - |a1,a2,a4|
= -2*(-5) - 2
= 8.
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