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a) Tính \(\sin \alpha ,cos\alpha \) theo \(\tan \dfrac{\alpha }{2} = t\)
b) Hãy tính \(\dfrac{{1 – \cos \alpha }}{{\sin \alpha }} + \dfrac{1}{{\tan \alpha }} + 4\sin \alpha \) theo \(\tan \dfrac{\alpha }{2} = t\).
a)
\(\begin{array}{l}\sin \alpha = 2\sin \dfrac{\alpha }{2}\cos \dfrac{\alpha }{2}\\ = 2\tan \dfrac{\alpha }{2}{\cos ^2}\dfrac{\alpha }{2} = \dfrac{{2t}}{{1 + {t^2}}}\end{array}\) (giả sử \(\cos \dfrac{\alpha }{2} \ne 0\))
\(\begin{array}{l}\cos \alpha = 2{\cos ^2}\dfrac{\alpha }{2} – 1\\ = \dfrac{2}{{1 + {{\tan }^2}\dfrac{\alpha }{2}}} – 1 = \dfrac{{1 – {t^2}}}{{1 + {t^2}}}\end{array}\) (giả sử \(\cos \dfrac{\alpha }{2} \ne 0\))
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b) Khi \(\sin \alpha \cos \alpha \ne 0\), ta có
\(\dfrac{{1 – \cos \alpha }}{{\sin \alpha }} + \dfrac{1}{{\tan \alpha }} + 4\sin \alpha = \dfrac{1}{{\sin \alpha }} + 4\sin \alpha \)
Vậy khi \(t = \tan \dfrac{\alpha }{2} \ne 0\) và \({t^2} \ne 1\), ta có
\(\dfrac{{1 – \cos \alpha }}{{\sin \alpha }} + \dfrac{1}{{\tan \alpha }} + 4\sin \alpha = \dfrac{{{t^4} + 18{t^2} + 1}}{{2t\left( {1 + {t^2}} \right)}}\)