The lengths of the sides of a rectangular parallelepiped are 11,12,13. Then the angle between two diagonals out of four diagonals is

If  OA¯,OB¯ , OC¯are considered as coordinate axes respectively such     that OA=13,OB=12,OC=11, Direction ratios of two diagonals AF¯,BE¯are  13,−12,−11 and 13,−12,11respectively.If  θbe the angle between the above two diagonals thencosθ=a1a2+b1b2+c1c2a12+b12+c12a22+b22+c22=169+144−121169+144+1212=192434=96217Therefore, θ=arccos96217