6) a, Đơn giản các biểu thức sau:

\(A=\dfrac{sin^4a-cos^4a+cos^2a}{2\left(1-\cos a\right)}\\ B=\dfrac{\sin\alpha+sin3\alpha+sin5\alpha+sin7\alpha}{cos\alpha+cos3\alpha+cos5\alpha+cos7\alpha}\\ C=\dfrac{1-2cos2x+cos4x}{1+2cos2x+cos4x}\\ D=\left(\dfrac{1+cos\beta}{1-cos\beta}\right)tan^2\dfrac{\beta}{2}+sin^2\beta-2\)

b, Chứng minh:\(\dfrac{cosx+sinx}{sin^3x}=1+\cot x+\cot^2x+\cot^3x\)

Giúp với ạGiúp với ạ

The bag was found later. The robbers put the money in it.

-> The bag in________