23. \(\left(\frac{2}{3}a-\frac{1}{2}\right)^2\)

\(\left(\frac{2}{3}a-\frac{1}{2}\right)^2=\left(\frac{2a}{3}\right)^2-2\cdot\frac{2a}{3}\cdot\frac{1}{2}+\left(\frac{1}{2}\right)^2=\frac{4a^2}{9}-\frac{2a}{3}+\frac{1}{4}\).

Ответ: \(\frac{4a^2}{9}-\frac{2a}{3}+\frac{1}{4}\).