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Question : Let ABC be an equilateral triangle and AD perpendicular to BC, Then AB2 + BC2 + CA2 =?

Option 1: 2AD2

Option 2: 3AD2

Option 3: 4AD2

Option 4: 5AD2


Team Careers360 16th Jan, 2024
Answer (1)
Team Careers360 24th Jan, 2024

Correct Answer: 4AD2


Solution :
Here ABC is an equilateral triangle with side length $a$ and AD $\perp$ BC.
So, BD = CD = $\frac{a}{2}$
In $\triangle$ABD, AB2 = BD2 + AD2
⇒ $a^2=(\frac{a}{2})^2+$ AD2
$\therefore$ AD = $\frac{\sqrt3}{2}a$ = $\frac{\sqrt3}{2}$AB = $\frac{\sqrt3}{2}$BC = $\frac{\sqrt3}{2}$AC [$\because$ AB = BC = AC = $a$]
⇒ AB = $\frac{2}{\sqrt3}$ AD, BC = $\frac{2}{\sqrt3}$ AD, and AC = $\frac{2}{\sqrt3}$ AD
$\therefore$ AB2 + BC2 + AC2 = AD2 ($\frac{4}{3}+\frac{4}{3}+\frac{4}{3}$) = 4AD2
Hence, the correct answer is 4AD2.

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