→a×→b=→b×→c
→a×→b=−→c×→b
∴→a×→b=−→c×→b=0
(→a+→c)×→b=0
∴→a×→b≠0,→b×→c≠0,→a,→b,→c are non-zero vectors. (→a+→c)≠→0
Hence, →a+→c is parallel to →b.
∴→a+→c=k→b