Let →a,→c be unit vectors and |→b|=4. The angle between →a and →c is cos−1(14). Then the positive integral value of λ such that →b−2→c=λ→a is
Let →a,→b,→c be three vectors such that |→a|=|→c|=1;|→b|=4 and |→b×→c|=√15. If →b−2→c=λ→a then a value of λ is