When to Use Maxima or Minima Equations in Thin-Film Interference?

[VitaX]

Homework Statement

[PLAIN]http://img135.imageshack.us/img135/6027/physicsch3569.png

Homework Equations

2L = (m + 0.5) (λ/n) for m = 0, 1, 2, ... (maxima - bright film in air)
2L = m(λ/n) for m = 0, 1, 2, ... (minima - dark film in air)

The Attempt at a Solution

Bright
2L = (10 + 0.5)(630E-9/1.5)
L = 2.205E-6 m

Dark
2L = 9(630E-9/1.5)
L = 1.89E-6 m

Is this the right approach? I was thinking the change in film thickness would be the difference between the two L values. But the book gives the answer as 1.89 E-6 m. Which is what I got for the Dark set of fringes. So was I supposed to negate the bright ones or did I use the wrong approach here?

 

[VitaX]

Someone told me to use the maxima equation with values of m being 0 and 9 and take the difference. I get the answer in the book then. But, how do you know when to use the maxima or minima equations for questions like this? I understand why we using 0 to 9 as the m values instead of 0 to 10, because 0-9 is 10 fringes. At least I think that's the reasoning behind it.