Đáp án:
$\begin{array}{l}7. \ 16a^2-(a^2+4)^2\\=(4a-a^2-4)(4a+a^2+4)\\=-(a-2)^2(a+2)\\8. \ \dfrac{1}{4}(a+1)^2-\dfrac{4}{9}(a-2)^2\\=\dfrac{1}{36}(9(a+1)^2-16(a-2)^2)\\=\dfrac{1}{36}(3(a+1)-4(a-2))(3(a+1)+4(a-2))\\=\dfrac{1}{36}(3a+3-4a+8)(3a+3+4a-8)\\=\dfrac{1}{36}(-a+11)(7a-5)\\9. \ \text{Không phân tích được.}\\*)\text{ Sửa:} \ (x^2+xy^2)^2-(y^2+xy)^2\\=(x^2+xy^2-(y^2+xy))(x^2+xy^2+(y^2+xy))\\=(x^2+xy^2-y^2-xy)(x^2+xy^2+y^2+xy)\end{array}$