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
¯b2=(2¯c+λ¯a)2⇒|¯b|2=4¯c2+λ2¯a2+4λ(¯a.¯c)
⇒16=4+λ2+λ (¯a.¯c=14)
λ2+λ−12=0
(λ+4)(λ−3)=0⇒λ=−4,3
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