Ответ:

  1. \(\left(\frac{x}{2}-\frac{y}{3}\right)^2=\frac{x^2}{4}-\frac{xy}{3}+\frac{y^2}{9}\).
  2. \(\left(\frac{a}{4}+\frac{b}{3}\right)^2=\frac{a^2}{16}+\frac{ab}{6}+\frac{b^2}{9}\).
  3. \(2\frac13=\frac73\), \(1\frac12=\frac32\); поэтому \(\left(2\frac13m+1\frac12n\right)^2=\left(\frac73m+\frac32n\right)^2=\frac{49}{9}m^2+7mn+\frac94n^2\).

Ответ: \(\frac{x^2}{4}-\frac{xy}{3}+\frac{y^2}{9};\ \frac{a^2}{16}+\frac{ab}{6}+\frac{b^2}{9};\ \frac{49}{9}m^2+7mn+\frac94n^2\).