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如图,在直三棱柱ABC-A_1B_1C_1中,E,F分别是A_1B,A_1C的中点,点D在B_1C_1上,A_1D⊥B_1C.
As shown in the figure, in the straight triangle ABC-A_1B_1C_1, E and F are the midpoints of A_1B and A_1C respectively, the points D are on B_1C_1 and A_1D⊥B_1C.