wave optics doubt...help

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let intensity at s₃,s₄ is I₁
as intensity at p=I so I₁=I/4
now let intensity at s1,s2=i2
i1=4*i2*cos²(Δφ/2)
Δφ=2π/λ*Δx=2π/λ*d(Dλ/2d)/2D=π/2
=>i2=i1/2=I/8
this is the intensity at s1

(a)as z=Dλ/d so intensity at s3=4*I/8*cos²(Δφ/2)
now Δφ=π,so intensity at s4,s3=0
hence intensity at P=0
(b)as z=3Dλ/2d so intensity at s3,s4=4*I/8*cos²(Δφ/2)
now Δφ=3π/2 so intensity at P=I
in the same method we can do the last one