[線代] 一題向量
A X (B X C) = (A˙C)B-(A˙B)C
下面是我的證法:
(B X C)_i = sigma (e_ijk)(B_j)(C_k) = D_i
jk
(A X D)_i = sigma (e_ilm)(A_l) sigma (e_ijk)(B_j)(C_k)
lm jk
= sigma (e_ilm)(e_ijk)(A_l)(B_j)(C_k)
lmkj
= sigma [(o_lj)(o_mk)-(o_lk)(o_mj)](A_l)(B_j)(C_k)
lmkj
when j=l, m=k; l=k, j=m
=> sigma [(A_j)(B_j)(C_k) - (A_l)(B_j)(C_l)]
jk
= (A˙B)C-(A˙C)B
差一個負號,為什麼會這樣??
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