When to use destructive or constructive equation for young's DS exp.

[Vickyvicksss]

Homework Statement

Light of wavelength 646 nm passes through a slit 5.50 × 10^-6 m wide and falls on a screen that is 2.01 m away. What is the distance on the screen from the center of the central bright fringe to the third dark fringe on either side?

Homework Equations

destructive: (m+1/2)λ=dsin(θ)
constructive: (m)λ=dsin(θ)
y=Ltanθ

The Attempt at a Solution

You're supposed to use the constructive equation, where m=3. Why don't we use destructive equation? (Since we're interested in the center to dark fringe at m=3)

 

[Simon Bridge]

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslit.html
If the slit width is a, then derive the condition for a minima to occur using pairs of rays separated by a/2.

 

[Vickyvicksss]

Helpful link! thank you :)