Given: In figure, the perpendiculars AD, BE and CF drawn from the vertices A, B and C respectively of ∆ABC are equal.
To Prove: ∆ABC is an equilateral triangle.
Proof: In right triangles BEC and CFB,
Hyp. BC = Hyp. CB | Common
Side BE = Side CF | Given
∴ ∆BEC ≅ ∆CFB
| RHS congruence rule
∴ ∠BCE = ∠CBF ...(1) | CPCT
In ∆ADB and ∆ADC,
∠ADB = ∠ADC (Each = 90°)
∠ABD = ∠ACD | From (1)
AD = AD | Common
∴ ∆ADB ≅ ∆ADC
| AAS congruence rule
∴ AB = AC ...(2) | CPCT
Similarly, we can show that
AB = BC ...(3)
and AC = BC ...(4)
From (2), (3) and (4), we get,
AB = BC = CA
⇒ ∆ABC is an equilateral triangle.