Answer
The path difference must be $~~(\frac{2n+1}{2})~\lambda~~$ where $n$ is an integer.
Work Step by Step
On page 1051, the text states, "A phase difference of 0.5 wavelength puts two waves exactly out of phase."
Therefore, the path difference must be $~~(n~\lambda+\frac{\lambda}{2})~~$ where $n$ is an integer.
Then:
path difference = $(n~\lambda+\frac{\lambda}{2}) = (\frac{2n+1}{2})~\lambda~~$ where $n$ is an integer.
