Coordinate planes and the planes \( \pi_1, \pi_2, \pi_3 \), which are respectively parallel to YZ, ZX, XY planes at distances a, b, c, form a rectangular parallelepiped. If \( d_1 \) is a diagonal of the face on XY-plane not passing through origin and \( d_2 \) is diagonal of plane \( \pi_2 \), then the cosine of the angle between \( d_1 \) and \( d_2 \) is: