The straight line B 1P is perpendicular to the plane BC1d.

So DP is the projection of DB 1 on the plane BC1d.

Therefore ∠B 1DP is the included angle between DB 1 and plane BC1d.

Because surface BC 1D// surface EFGHKL

Therefore, ≈b 1DP is the included angle between DB 1 and the surface EFGHKL.

I hope I can help you.