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, []Isosceles triangle and the length of parallel segments / Origical theorem(GC World : Various problems ,Is the useless one useless?)
There is a isosceles triangle ABC. Point P is on the base BC.
Two lines which are parallel to AB and AC respectively and intersect on P are drawn.
The intersections of parallel lines with AB and AC are point Q and R respectively.
Then PQ + PR is constant.
GC/Win