Đáp án:

$\dfrac{49}{25}$.

Giải thích các bước giải:

$\dfrac{\left(\dfrac7{2^9}-\dfrac{14}{2^{11}}+\dfrac{21}{768}\right)^2}{\left(\dfrac5{2^9}-\dfrac{20}{2^{12}}+\dfrac{25}{1280}\right)^2}\\=\dfrac{\left(\dfrac7{2^9}-\dfrac{2.7}{2^{11}}+\dfrac{3.7}{3.2^8}\right)^2}{\left(\dfrac5{2^9}-\dfrac{2^2.5}{2^{12}}+\dfrac{5.5}{5.2^8}\right)^2}\\=\dfrac{\left(\dfrac7{2^9}-\dfrac7{2^{10}}+\dfrac7{2^8}\right)^2}{\left(\dfrac5{2^9}-\dfrac5{2^{10}}+\dfrac5{2^8}\right)^2}\\=\left(\dfrac{\dfrac7{2^9}-\dfrac7{2^{10}}-\dfrac7{2^8}}{\dfrac5{2^9}-\dfrac5{2^{10}}+\dfrac5{2^8}}\right)^{\large2}\\=\left(\dfrac{7.\!\left(\dfrac1{2^9}-\dfrac1{2^{10}}-\dfrac1{2^8}\right)}{5.\!\left(\dfrac1{2^9}-\dfrac1{2^{10}}+\dfrac1{2^8}\right)}\right)^{\large2}\\=\left(\dfrac75\right)^2\\=\dfrac{49}{25}$