Co ordinate in space

(I) first of all you have to suppose the direction cosines of a line,
(ii) find the direction cosines of all the diagonals of a cube.
(iii) then given angles makes the angles with four diagonals of a cube .
(iv) Find the angles with four diagonals of a cube.
(v) take l.H.S. and calculate.
finally we get required answer.