The correct answer is (a) 60°
To elaborate: The nulls of the N- element array is given by
\(θ_n=cos^{-1}(\frac{λ}{2πd} \left[-β±\frac{2πn}{N} \right])=cos^{-1}(\frac{λ}{2πd} \left[±\frac{2πn}{N} \right])\)
⇨ \(θ_n=cos^{-1}(\frac{λ}{2π(λ/2)} \left[±\frac{2πn}{N} \right])=cos^{-1} (±\frac{2n}{4})=cos^{-1}(±\frac{n}{2})\)
⇨ \(n=1 (first ,null) cos^{-1} (±\frac{n}{2})=cos^{-1} (±\frac{1}{2})=60° or 120°. \)
