draw two lines, AB and AC

with any point, D, 'between' them

drop perpendiculars from D to AB (at F) and to AC (at E)

join E to F (the feet of the perpendiculars)

drop a perpendicular from A to EF (at G)

set this up (e.g. with GeoGebra)

establish, by measuring and dragging D around, that angle GAE = angle FAD

prove it

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