If →a×(→b×→c) is perpendicular to (→a×→b)×→c then, we may have,
→a.→c=0
→a×(→b×→c)=(→a.→c)→b−(→a.→b)c(→a×→b)×→c=−(→c.→b)→a+(→a.→c)→b
we have been given,
(→a×(→b×→c)).((→a×→b)×→c)=0⇒[(→a.→c)→b−(→a.→b)→c].[(→a.→c)→b−(→c.→b)→a]=0⇒(→a.→c)2|→b|2−2(→a.→c)(→b.→c)(→a.→b)+(→a.→b)(→b.→c)(→c.→a)=0⇒(→a.→c)2|→b|2=(→a.→c)(→a.→b)(→b.→c)⇒(→a.→c)((→a.→c)(→b.→b)−(→a.→b)(→b.→c))=0⇒→a.→c=0 or (→a.→c)|→b|2=(→a.→b)(→b.→c)