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edit approved Nov 29, 2012 at 10:58

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TrigExpand@Tan[x + y]

gives

$\frac{\sin (x) \cos (y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}+\frac{\cos (x) \sin (y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}$$\frac{\sin (x) \cos (y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}+\frac{\cos (x) \sin(y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}$

but my excepted result isI expected

$\frac{\tan (x)+\tan (y)}{1-\tan (x) \tan (y)}$

On theEvaluating TrigExpand with arguments $\tan (x-y)$ or $\tan (3*x)$ is also the case,returns results in terms of sine and cosine. What should I do to get results in terms of the tangent?

TrigExpand@Tan[x + y]

gives

$\frac{\sin (x) \cos (y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}+\frac{\cos (x) \sin (y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}$

but my excepted result is

$\frac{\tan (x)+\tan (y)}{1-\tan (x) \tan (y)}$

On the $\tan (x-y)$ or $\tan (3*x)$ is also the case, What should I do?

TrigExpand@Tan[x + y]

gives

$\frac{\sin (x) \cos (y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}+\frac{\cos (x) \sin(y)}{\cos (x) \cos (y)-\sin (x) \sin (y)}$

but I expected

$\frac{\tan (x)+\tan (y)}{1-\tan (x) \tan (y)}$

Evaluating TrigExpand with arguments $\tan (x-y)$ or $\tan (3*x)$ also returns results in terms of sine and cosine. What should I do to get results in terms of the tangent?