In the cube abcd-a1b1c1d1, e and F are the midpoint of dd1 and DB, respectively

In the cube abcd-a1b1c1d1, e and F are the midpoint of dd1 and DB, respectively


Connect BD1. In the triangle dbd1, because E.F is the midpoint of dd1 and DB respectively, EF / / BD1 and BD1 are in the face abc1d1, so EF / / abc1d1