Ta có: $A=\frac{2(1-2sin^2\frac{x}{2})}{2sin^2\frac{x}{2}}=\frac{1-2sin^2\frac{x}{2}}{sin^2\frac{x}{2}}$Chia cả tử và mẫu cho $cos^2\frac{x}{2}$$\Rightarrow A=\frac{1+tan^2\frac{x}{2}-2tan^2\frac{x}{2}}{tan^2\frac{x}{2}}=\frac{1-tan^2\frac{x}{2}}{tan^2\frac{x}{2}}$$=\frac{1-\frac{16}{25}}{\frac{16}{25}}=\frac{9}{16}$
Ta có: $A=\frac{2(
cos^2\frac{x}{2}-sin^2\frac{x}{2})}{
(cos\frac{x}{2
}-sin\frac{x}{2}
)^2}=\frac{2
(cos\frac{x}{2}+sin\frac{x}{2}
)}{
cos\frac{x}{2}-sin\frac{x}{2}}$Chia cả tử và mẫu cho $cos\frac{x}{2}$$\Rightarrow A=\frac{2
+2tan\frac{x}{2}}{1-tan\frac{x}{2}}=
0$