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) cos−1(1731) C) 300 D) 900 Answer: Option AExplanation:→AO=ˆi+2ˆj+ˆk→AC=−2ˆi−ˆj+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)+1×1√1+4+1√4+1+1−36=−12=cos1200 θ=1200