Đáp án:

 `bbB`

Giải thích các bước giải:

 Câu `7:`

`P=\frac{cos\alpha+2cos3\alpha+cos5\alpha}{sin\alpha+2sin3a+sin5a}`

`=\frac{(cos\alpha+cos5\alpha)+2cos3\alpha}{(sin\alpha+sin5\alpha)+2sin3a}`

`=\frac{2.cos\frac{\alpha+5\alpha}{2}.cos\frac{\alpha-5\alpha}{2}+2cos3\alpha}{2.sin\frac{\alpha+5\alpha}{2}.cos\frac{\alpha-5\alpha}{2}+2sin3\alpha}`

`=\frac{2cos3\alpha.cos(-2\alpha)+2cos3\alpha}{2sin3\alpha.cos(-2\alpha)+2sin3\alpha}`

`=\frac{2cos3\alpha.[1+cos(-2\alpha)]}{2.sin 3\alpha.[1+cos(-2\alpha)]}`

`=\frac{cos3\alpha}{sin3\alpha}`

`=cot3\alpha`

Vậy `P=cot3\alpha`

`->` Ta chọn đáp án: `bbB`