In videos destructive interference is given (2n+ 1)lambda/2 where as in board book it is given by (2n - 1)lambda/2. Which of these is correct for destructive interference?

Q. Destructive interference formula is given as (2n+1)lambda/2 in NCERT. But I found out to be as (2n-1)lambda/2. Which one is correct?

Q. Match the condition for the type of interference:
1. Constructive I. $(2n+1)\frac{\lambda }{2}$
2. Destructive II. $\left(2n\right)\lambda$

Q. Match the condition for the type of interference:
1. Constructive I. $(2n+1)\frac{\lambda }{2}$
2. Destructive II. $\left(2n\right)\lambda$

Q. Two coherent sources $S_1$ and $S_2$ which emit sound of wavelength $\lambda$ in phase are separated by $2\lambda$. A circular wire of large radius is placed in such a way that $S_1{S}_2$ lies in its plane and the middle point of $S_1{S}_2$ is at the centre of the wire as shown below. Find the angular position $\theta$ (in first quadrant) on the wire for which destructive interference takes place.
[Assume the radius of ring $R>>2\lambda$]

Q. A thin film of liquid polymer, $n=1.25$ coats a slab of Pyrex, $n=1.50$. White light is incident perpendicularly on the film. In the reflection, full destructive interference occurs for $\lambda =600nm$ and full constructive interference occurs for $\lambda =700nm$. What is the thickness of the polymer film?