# A tetrahedron has vertices at (0, 0, 0) , (1,2,1),  (2,1,3) and  (-1,1,2). Then the angle between the faces  and  will be  Option 1) Option 2) Option 3) Option 4)

As we learnt in

Ange between two lines in terms of direction cosines and direction ratios -

If two lines having direction ratios a1,b1,c1 and a2,b2,c2 then the angle between them is given by

If two lines have direction ratios as l1,m1,n1 and l2,m2,n2 then the angle betwee them is given by

-

Vector perpendicular to face OAB=

Similarily, vector perpendicular to face ABC=

Angle between faces

(Using dot product)

Option 1)

Incorrect option

Option 2)

Incorrect option

Option 3)

Incorrect option

Option 4)

Correct option

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