In triangle ABC, ad is perpendicular to BC and D, be is the middle line, and the angle EBC is 30 degrees

If a straight line EF is made through point E and perpendicular to BC, then EF is parallel to ad, because point E is the midpoint, so EF = 1 / 2ad, and because the angle EBC = 30 degrees, ad is perpendicular to BC, so EF = 1 / 2be, ad = be can be deduced

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