28.8. a) (0,2x 0,5a)²; 1 2 6)=m+3n; 4 г) (10с + 0,1у)² .

a) $$(0.2x - 0.5a)^2 = 0.04x^2 - 0.2xa + 0.25a^2$$.

б) $$(\frac{1}{4}m + 3n)^2 = \frac{1}{16}m^2 + \frac{3}{2}mn + 9n^2$$.

г) $$(10c + 0.1y)^2 = 100c^2 + 2cy + 0.01y^2$$.

Ответ: a) $$0.04x^2 - 0.2xa + 0.25a^2$$, б) $$\frac{1}{16}m^2 + \frac{3}{2}mn + 9n^2$$, г) $$100c^2 + 2cy + 0.01y^2$$.