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 anles

b) all r right angles

 

c) atleast one is acute angle

 

d) none of dis

ok lets eliminate options

a option. obviously false. the sum of the 3 vectors would surely not be unit vector.

b option. again false. take unit vectors along the three axes and verify. magnitude will be root 3.

now take a simple case.

vector a along x direction

vector b along y direction

vector c along -x direction.

this satisfies all cinditions.

so then option c is correct.

from the abive case we also see that:

a along x dirn

b making acute angle with y dirn

c making acute angle with -x dirn will also satisfy conditions for some values of angles.

answer is c

Yeah, the answer is (c)...Here's a particular case...