18) (an.a²): a1004 at

18) $$\frac{(a^{5n} \cdot a^2) : a^{10}}{a^3} = a^4$$ $$\frac{a^{5n+2} : a^{10}}{a^3} = a^4$$ $$\frac{a^{5n-8}}{a^3} = a^4$$ $$a^{5n - 8 - 3} = a^4$$ $$a^{5n - 11} = a^4$$ Следовательно, $$5n - 11 = 4$$ $$5n = 4 + 11$$ $$5n = 15$$ $$n = 15 : 5$$ $$n = 3$$

Ответ: 3