A. \(\tan \left( {\alpha + \beta } \right) = 2\tan \beta .\)
B. \(\tan \left( {\alpha + \beta } \right) = 3\tan \beta .\)
C. \(\tan \left( {\alpha + \beta } \right) = 4\tan \beta .\)
D. \(\tan \left( {\alpha + \beta } \right) =5\tan \beta .\)
C
Ta có
\(\begin{array}{l}
5\sin \alpha = 3\sin \left( {\alpha + 2\beta } \right) \Leftrightarrow 5\sin \left[ {\left( {\alpha + \beta } \right) - \beta } \right] = 3\sin \left[ {\left( {\alpha + \beta } \right) + \beta } \right]\\
\Leftrightarrow 5\sin \left( {\alpha + \beta } \right)\cos \beta - 5\cos \left( {\alpha + \beta } \right)sin\beta = 3\sin \left( {\alpha + \beta } \right)\cos \beta + 3\cos \left( {\alpha + \beta } \right)sin\beta \\
\Leftrightarrow 2\sin \left( {\alpha + \beta } \right)\cos \beta = 8\cos \left( {\alpha + \beta } \right)sin\beta \Leftrightarrow \frac{{\sin \left( {\alpha + \beta } \right)}}{{\cos \left( {\alpha + \beta } \right)}} = 4\frac{{\sin \beta }}{{\cos \beta }} \Leftrightarrow \tan \left( {\alpha + \beta } \right) = 4\tan \beta
\end{array}\)
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