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## Equilateral triangle example

Question: ABC is an equilateral triangle. AD is perpendicular to BC at point D. Extended AB to the point E to make EB = DB. Prove AD = ED.

- Proof:
- Because ABC is an equilateral triangle (given),
- so, angle ABC = angle C = angle CAB = 60
^{o} - (Each interior angle in an equilateral triangle is 60 degrees.)
- Because AD perpendicular to BC at point D (given),
- so, DB = CD and
- angle BAD = angle CAD = (1/2) angle CAB = (1/2) 60
^{o}= 30^{o} - (The altitude of an isosceles triangle bisects its vertex and its base.)
- so, BD = DC = (1/2)BC
- Because angle ABD is an external angle of the triangle BED,
- so, angle ABD = angle E + angle BDE.
- (external angle of a triangle theorem)
- Because BE = BD (given),
- so, angle E = angle BDE
- (In a triangle, congruent sides opposite congruent angles.)
- angle E = angle BDE = (1/2) angle ABD = (1/2) 60
^{o}= 30^{o} - In triangle AED, because angle E = angle DAE, so ED = AD
- (In a triangle, congruent angles opposite congruent sides.)