Даю 20Балов!!! tg(4а)=?, если tg=1/6 по формуле tg(2a)=2tg(a)/(1-tg^2(a))

Решите задачу:

$tga= \frac{1}{6}\\\\tg2a=\frac{2\cdot tga}{1-tg^2a} = \frac{2\cdot \frac{1}{6}}{1-\frac{1}{36}} = \frac{36}{3\cdot 35} = \frac{12}{35} \\\\tg4a= \frac{2\cdot tg2a}{1-tg^22a} = \frac{2\cdot \frac{12}{35}}{1- \frac{144}{1225} } = \frac{1225\cdot 24}{35\cdot 1081} = \frac{35\cdot 24}{1081} = \frac{840}{1081}$