Ta có
$A = \dfrac{125^3.15^3 + 15^3.5^9}{108.5^{12}}$
Phân tích ra ta có
$A = \dfrac{(5^3)^3 . (3.5)^3 + (3.5)^3 . 5^9}{2^2.3^3.5^{12}}$
$= \dfrac{5^9.5^3.3^3 + 5^9.5^3.3^3}{2^2.3^3.5^{12}}$
$= \dfrac{5^{12}. 3^3 + 5^{12}.3^3}{2^2.3^3.5^{12}}$
$= \dfrac{2.3^3.5^{12}}{2^2.3^3.5^{12}}$
$= \dfrac{2}{2^2} = \dfrac{1}{2}$
Vậy $A = \dfrac{1}{2}$.