If a,b,c are three unit vectors such that a+b+c is also unit vector and @,@2,@3 are angles  between vectors a,b; b,c and c,a respectively then among @,@2,@3 a) all r acute b) all r 90 c) atleast one is acute d) none of dis

ITS 2@ OR @/2

Answer is d

Mod(a+b+c)=1.

sqaring on both sides.

1+1+1+2(cos@+Cos2@+Cos3@)=1   (mod a=mod b=mod c=1)

             Cos@+Cos2@+Cos3@=-1

So atleast one angle is obtuse.