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¢éͤÇÒÁà´ÔÁà¢Õ¹â´Â¤Ø³ legendaryfool
¨§ËÒ¤èҢͧ $cot 9\left(\frac{1}{cot 9-3tan9}+\frac{3}{cot 27-3tan27}+\frac{9}{cot 81-3tan81}+\frac{27}{cot 243-3tan243}\right)$
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à¹×èͧ¨Ò¡ $\frac{1}{\cot A - 3 \tan A} = \frac{\tan A}{1 - 3\tan^2A} ... (*)$
¨Ò¡ $\tan 3A = \frac{3\tan A - \tan^3A}{1 - 3\tan^2A}$
´Ñ§¹Ñé¹ $3\tan 3A = \frac{9\tan A - 3\tan^3A}{1 - 3\tan^2A} = \frac{8\tan A + \tan A - 3\tan^3A}{1 - 3\tan^2A} = \frac{8\tan A}{1 - 3\tan^2A} + \tan A$
¨Ðä´é $\frac{\tan A}{1 - 3\tan^2A} = \frac{1}{8}(3\tan 3A - \tan A)$