Đáp án:
`{tan 2\alpha}/{tan 4\alpha -tan2\alpha}=cos 4alpha`
Giải thích các bước giải:
`{tan 2\alpha}/{tan 4\alpha -tan2\alpha}`
\(=\dfrac{\tan 2\alpha}{\dfrac{2\tan 2\alpha}{1-\tan^22\alpha}-\tan 2\alpha}\)
`={1-\tan^2 2alpha}/{1+tan^2 2\alpha}`
`=cos 4alpha`
Vậy `{tan 2\alpha}/{tan 4\alpha -tan2\alpha}=cos 4alpha`