1) $$20a^3 \cdot (5a)^2$$

$$20a^3 \cdot (5a)^2 = 20a^3 \cdot 25a^2 = 20 \cdot 25 \cdot a^3 \cdot a^2 = 500a^{3+2} = 500a^5$$

Ответ: $$500a^5$$