18) (an.a²): a10 a³

18) $$\frac{(a^{5n} \cdot a^2) : a^{10}}{a^3} = a^0$$ $$\frac{a^{5n+2} : a^{10}}{a^3} = a^0$$ $$\frac{a^{5n+2-10}}{a^3} = a^0$$ $$\frac{a^{5n-8}}{a^3} = a^0$$ $$a^{5n-8-3} = a^0$$ $$a^{5n-11} = a^0$$ $$5n-11 = 0$$ $$5n = 11$$ $$n = \frac{11}{5}$$

Ответ: $$n = \frac{11}{5}$$