Given equation is `cos^(-1)(cos x)=(n-x)/n,xge0`
or `cos^(-1)(cos x)=1-x/n,xge0`
For seven roots of equatio, the graphs of `y=cos^(-1)(cos s)" and "y=1-x/n` intersect seven times.
For seven roots, the line `y=1-x/n` must meet the x-intercept is between `6pi" and "8pi`.
`:. n in (6pi, 8pi)`