19) (a20.a): an (a2)2

19) $$\frac{(a^{20} \cdot a^4) : a^{4n}}{(a^2)^2} = a^4$$ $$\frac{a^{24} : a^{4n}}{a^4} = a^4$$ $$\frac{a^{24-4n}}{a^4} = a^4$$ $$a^{24-4n-4} = a^4$$ $$a^{20-4n} = a^4$$ $$20-4n = 4$$ $$-4n = 4-20$$ $$-4n = -16$$ $$n = \frac{-16}{-4}$$ $$n = 4$$

Ответ: $$n = 4$$