028.13. a) a-116 b)²; 14 6) (0,9x + 1);

a) $$(\frac{2}{3}a - \frac{1}{14}b)^2 = \frac{4}{9}a^2 - \frac{1}{21}ab + \frac{1}{196}b^2$$.

б) $$(0.9x + \frac{13}{27}y)^2 = 0.81x^2 + \frac{26}{30}xy + \frac{169}{729}y^2$$.

Ответ: a) $$\frac{4}{9}a^2 - \frac{1}{21}ab + \frac{1}{196}b^2$$, б) $$0.81x^2 + \frac{26}{30}xy + \frac{169}{729}y^2$$.