Here, α = β = γ
⇒ cosα = cosβ = cosγ
⇒ l = m = n = p(say)
Now, we know that -
l2 + m2 + n2 = 1
⇒ p2 + p2 + p2 = 1
⇒ 3p2 = 1
∴ the direction cosines of r⃗ are -
⇒ r⃗ = |r⃗|( l î + m ĵ + n k̂)
Now, multiplying and dividing it by √3
⇒ r⃗ = ±2√ 3 ( î + ĵ + k̂)