What is the expression for the angular width of the central maximum in single slit diffraction? How to derive it?

on the either side of the central maxima, there are first secondary minimas.

the angle of the first secondary minima above the central maxima is θ₁= λ/d and

1^st secondary minima on the other side will be θ₁¹= -1λ/d.

the angle between these two minima will b the angular width of the central maxima i.e 2 λ/d