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plz help me in the following problem?

"The electric field at point P due to N slits is,

E (r, theta, t) = A(r) sin (N delta phi/2) cos(kr-wt)

_____________

sin (delta phi/2)

where delta phi=kdsin (theta) , "d" being the spacing between the adjacent slits. let N=4

(a) Find two angles on either side of theta=0 at which there is an interference minima.

(b) Find the spacing between the minima compare to the case of only two slits of separation "d".

Now i have solve its (a) part but i just cant solve its (b) part.

Can u plz help me in this plzzzzzzzzzzzzzzzzz.

Thanks in advance!

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# Plz help me in this light problem

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