1)

A tetrahedron has vertices at O(0,0,0) ,  A( 1,2,1)  , B (2,1,3)  and C (-1,1,2) . Then the angle between the faces OAB and ABC will be


A) 1200

B) cos1(1731)

C) 300

D) 900

Answer:

Option A

Explanation:

AO=ˆi+2ˆj+ˆk
AC=2ˆiˆj+ˆk 

 Angle between faces OAB  and ABC  = Angle between  AO and AC .

 If Q be the angle between AO and AC , then 

cosθ=AO.AC|AO||AC|

=1×(2)+2×(1)+1×11+4+14+1+1
36=12=cos1200

θ=1200