We just need to substitute in the value of nn and simplify.

\(\displaystyle{A}{n}={n}−{\frac{{{1}}}{{{2}}}}\)

\(\displaystyle∴{A}{11}={\left({11}\right)}−{\frac{{{1}}}{{{2}}}}\)

\(\displaystyle∴{A}{11}=\frac{{21}}{}\)

\(\displaystyle{A}{n}={n}−{\frac{{{1}}}{{{2}}}}\)

\(\displaystyle∴{A}{11}={\left({11}\right)}−{\frac{{{1}}}{{{2}}}}\)

\(\displaystyle∴{A}{11}=\frac{{21}}{}\)